Having had all three Mark V's I can express my sympathy to your dialing challenges. On The Gear Page site a poster named dead astronaut posted the following (that I agree with). He is talking about the 5/25 but it applies to the 90W as well. I don't think he will me mind me re-posting it here.
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From: DeadAstronaut on The Gear Page discussing dialing in a Mark V/25
PART 1: THE KNOBS
People have expressed this various ways, and to me, the most helpful explanation is to say that the knobs give you the amount and texture of your gain, whereas the graphic EQ cuts or boosts frequency bands in the amp's overall output. You might think of the preamp EQ section as a very powerful "shape" or "contour" control.
Gain: Like Mesa says, we're not just looking at raw gain quotient here, we're also dealing with a very powerful tone-shaping tool that moves from the brightness of a cranked BF/SF Fender through a medium-bright Tweedy crunch and all the way up to the mega-high-gain saturation that made Boogies famous. Ideally, you should set Gain with a pretty clear idea of the general tonal character you want. Heavy saturation, in my experience (and not just the V:25 combo), sets in around 2:00. I like it between 3:00 and 4:00, depending on the guitar, the part, etc.
You also don't want or need to turn Gain all the way up. Past about 4:30, it mostly adds noise, and you're in danger of turning some preamp tubes microphonic.
Treble: Here's one crucial thing about Mesa/Boogie EQ, and not just on the Marks – whichever knob/slider you set highest determines the dominant character of your tone, at least in that section of the EQ, and the other controls recede. Cranking the Mid knob up, for example, does a number of things other than increasing the midrange. It'll limit the effectiveness of the Treble and Bass knobs, and it'll add an extra band of cutting high mid frequencies near the top of the dial. Set knobs, therefore, not just each to itself, but with keen attention to the ratio between them.
The Treble knob acts, up to a certain point, as a kind of gate on the Gain knob, so you don't want to crank Gain and then cut Treble. (For example: if you want a big, muffled, doomy/stoner tone, you'd want to keep Treble reasonably high to get all the gain and then crank your Bass knob.) I personally find that Treble around 1:00/1:15 works very well. Also, like Gain, be careful about cranking Treble too high – maxed out settings at high volume can make your tubes start complaining.
Mid: Perhaps this isn't the case on other Boogie Marks, but on the V:25, the Mid knob isn't the rich, creamy kind – it's much more about cut, body, presence, and ability to hold your own in the mix. Lots of people scoop it, both here and in the GEQ; I hate that sound and think Mid right around 12:00, or just a touch above, works really well on most Mesa/Boogies. As aforementioned, Mid will also introduce some very cutting high-mid frequencies as you crank it past 2:00 or so, and those come with a corresponding increase in both volume and background nois.
Bass: Ah, the most controversial knob on Boogies. Consensus wisdom will tell you to keep it around 9:00 and probably even lower it from there. If you want that crazy-tight, metal-rhythm, kick-in-the-chest IIC+ sound, then yes, drop it as low as you like. If, however, you're looking for a thicker, richer, more liquid tone and don't especially need extreme tightness in the bass end, don't let anyone tell you where to set this control. For my needs (I'm aiming for a heavily saturated, fluid lead tone that's still articulate enough for complex chords, à la later Shawn Lane/Holdsworth's Boogie and Recto years/’70s Frank Zappa), I think Bass sounds great around 11:00. Come to think of it, Treble 1:00 / Mid 12:00 / Bass 11:00 is where I set my Dual Recto 25 combo, too, and it's just killer.
Presence: Again, not sure if this applies to all Marks or just more recent Mesa/Boogies, but on the Mark V (and the more recent versions of the Recto), Presence is NOT a strictly linear control. From about 11:00 upward, it does the familiar thing, adding a significant bandwidth of high-treble harmonics and some extra chirp on the pick attack. From 11:00 downward, however, Presence doesn't just remove or de-emphasize those frequencies, it actually adds compression and smoother, more singing gain texture. I keep the Presence on all my Mesas very low, like in the 7:30-9:00 range depending on guitar, musical purpose, etc. For me, this is one of the great Mesa/Boogie features: the warm, smooth, vocalized compression you get on high-gain leads with Presence around 7:30 and, in my case, a PRS on its neck pickup is among my very favorite guitar sounds in the known universe.
PART 2: THE GRAPHIC EQ
If the "normal" knobs exist to determine shape and saturation of gain, the graphic EQ is a bit like mastering your guitar tone: it enters your signal path after the TMB and Gain knobs, and it boosts or cuts particular bandwidths, not just the specific frequencies named beneath the sliders. (I don't know if Mesa's ever publicized the exact figures here, or if they're different for each Mark, but to me, it sounds like the middle 3 sliders have a much lower Q, i.e. affect a much wider bandwidth, than the highest and lowest sliders.)
First cautionary tale: You've no doubt seen plenty of Boogie Mark players using the "smile" or "V-scoop" on their GEQs. That's a fine sound for certain things; it's by no means "the best," or "necessary." Some well-meaning people will tell you that you've absolutely got to scoop the middle faders, especially the 750Hz slider (pardon if I use "slider" and "fader" interchangeably), and especially on thicker-sounding Marks like the IV or the Mark IV mode on the V. Not true. That's one sound among many; maybe you'll like it, maybe you won't, and maybe you'll love it with 2 guitars and hate it with 3 others, and likewise for speakers, cabs, rooms ...
Second cautionary tale: Different Boogie Marks have the lines on their graphic EQs in different places. My V:25 has middle, top, and bottom lines, but there's about 1/3" extra above the "top" and below the "bottom"; I know some other Marks go top, halfway-to-middle, middle, halfway-to-bottom, bottom. As with everything, ears first.
Third and MOST IMPORTANT cautionary tale: Just as with the pre-gain EQ knobs, the overall sonic character of this section of your Mark will be determined by which slider you turn highest. If you want a crushing, sludgy, low-middy sound, for instance, you're not going to get it by "tricking" the amp into some arcane combination of sliders with the low-mids set paradoxically quiet.
This, I think, is absolutely key: test each slider, decide which one you want to be the cornerstone of your tone, and then arrange the others beneath it in whatever way fills out a sound you like. That's the only "rule" I've found to hold true. Whatever slider is highest will be dominant, and if you find yourself doing some strange things with the others around it, so much the better – you've got your own sound now!
THE FADERS
80 Hz: Perhaps the single most common mistake I've seen in EQing Boogies, especially smaller combos without the natural thump of a 4x12 cabinet, is to crank the 80 Hz slider right from the start. As somebody who did this myself, believe me, you're bringing a lot of unnecessary grief into your life. An overused 80 slider will not make a combo sound like a full stack, it'll just make everything boomy, subby, and indistinct, and – most importantly – you'll drive yourself crazy wondering why your other EQ changes don't help, when the answer is that your 80 Hz slider is drowning them all out.
(Example: @fatbagg, who gets some of the best sounds on TGP, runs the Bass knob at 1 and the 80 slider nearly off on his Mark IIC+, but because he's playing it through a cab with darker-toned speakers chosen for that exact application, he gets possibly the greatest IIC+ lead sound I've ever heard.)
Setting each slider will be intensely personal, of course, and as I've mentioned, I'd encourage you to turn each of them way up and way down, leaving the others in the middle; that way, you'll understand the character of each slider, and you can decide which one you'd like to dominate your post-gain tone. I personally favor Mark IV mode on the lead channel of my V:25, and I keep 80 Hz just a slight bump above the middle line, maybe a third of the way between the middle and top. That gives me plenty of oomph without subbing or rattling everything out.
240 Hz: A touchy one, and very important to the character of your sound. Too little, and things get razor-thin and acquire a sibilant, fizzy attack (which might be what you want!); too much, and it's mud all day. Just right, however, and you get a beautiful low-mid richness that makes chords hit like piledrivers and adds major punch to your single-note lines. I like it bumped a bit above the center line, just a touch higher than 80 Hz.
750 Hz: This is the Command Center, lieutenants. No other slider does so much to define your tone, and simply sweeping it from top to bottom and stopping at any random point will make you think you've been fooled into playing a different amp. The lower you go, the thinner and more brutal the tone, but also the quieter and more washed-out sounding. This is one great key to the 750 slider: it has an enormous effect on overall amp volume, and without changing your master, you can go from "barely audible" to "too loud without earplugs" just via the 750.
On the other end, from the middle up, the 750 gives you thickness, richness, chordal crunch, and that legendary Mesa Mark "liquid lead" tone, but be careful: there is a VERY DEFINITE POINT, which will vary depending on your guitar, amp, speakers, etc., at which the 750 Hz slider veers over into pure nasal honk. Unless you're looking for a special effect in the studio, that's probably not your goal here. If you're looking for the kind of tones I favor, I'd suggest setting 750 both first and highest; I keep it slightly over halfway between the middle and top lines.
All hail 750 Hz. It's your best friend and, ill-used, can become your biggest headache.
2200 Hz: What I think of as "the crunch slider." This is where you really get the chunk and crackle out of your chords, but be careful: when set too high, 2200 will give you a more trebly version of that 750 nasal honk, and, very importantly, it will hyper-emphasize all the little accidental sounds you make while playing (fingers sliding along strings, thumb moving on the back of the neck, belt buckle tapping the vibrato-system cover, etc.). You want just enough crunch to stay, well, crunchy, and not enough for all that irritating stuff. I run mine a little bit over the center line – I'd have to check, but I think it's right at the point where the white line becomes visible under the bottom of the fader.
6600 Hz: The Mystery Slider. Now, some of you are going to turn up the 6600 slider, hear all that vicious seething sizzling treble, think F*ck yeah!, and that'll be all. For you: may you be blessed and depart in peace to love and serve the Boogie.
But for those of us who like a thicker, more fluid tone, the 6600 is a bit of a conundrum, or will seem like one until you finally figure it out.
As I've said god-knows-how-many-times, so much of the Boogie EQ process is about finding the knob/slider that gives your basic character, then dialing in good ratios of the other knobs/sliders. So when I first heard the 6600 Hz slider, I thought, "Well, don't need any of THAT," and turned it off. I then spent weeks struggling to get a tone that was creamy, thick, and fluid, but had enough cut and snarl to stay articulate and to keep the top end of my chords ringing. I messed with the Treble knob, with Presence, with 2200, even with turning off the GEQ altogether (not recommended), until I figured it out: mix just a little bit of 6600 Hz in there.
The sound of the 6600 slider on its own is less than delightful to me, but if I juice that range just a touch, I can keep the Treble where I like it, I don't have to change the Gain, the Presence can say in those low regions where it's providing compression, and – perhaps most crucially – I can keep all that rich midrange I love so much. All I had to do was move the 6600 slider up from the very, very bottom to the point where the bottom of the fader is covering the bottom white line. (For users of the old 5-line GEQ graphic, this would be 1/3rd to 1/2 between the bottom and middle-bottomm lines.)
Hallelujah! All my midrange, all my thickness, fluidity, and compression, and suddenly, the top end opened up, and I had chord articulation an clarity all day!
And that, brethren and ... sestren? ... is what I've learned about EQing a Mesa Boogie Mark, particularly the Mark V:25. I'll also add that I prefer to keep these settings for all 3 modes of the lead channel, because I like how they react differently: IV is my central big-chord/liquid-lead sound, IIC+ is tighter, more focused, and goes great with Teles and bolt-ons, and Xtreme (Extreme for you big-V users) gives me a crazy chordal roar and chug.
Thanks to anybody who read through this little discursus, and I sincerely hope I've saved at least one person some time and frustration, and made it more fun to play your Boogie. Thanks in particular to @fatbagg, @teofilrocks, and @De Batz for things they've taught me over the last few months.
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Settings Advice
What I think I'd suggest for a gainy tone is to set Gain first (always do that, because it really affects the EQ), then dial in the TMB tone stack and Presence control at moderate positions – which, for a Mark V, means something like Treble at 1 o'clock, Mid wherever you prefer it between 10:30 and 1:30, and Bass about 9:00. Set Presence wherever you tend to like it between 9 o'clock and noon.
Then I'd recommend setting the GEQ like this: presuming you want more low end than the very, very small amount you'll be getting, turn the 80 Hz slider most of the way up, say slightly below the top line. I think the next move, then, is to move the 750 Hz slider all the way up and down and figure out where you like it, because that middle GEQ slider is the single most determinative factor in your tone (other than the Gain control). Once you figure out where you want the very middle midrange, you can add or subtract little bits of the other 3 sliders. 240 Hz adds thickness turned up, clears out mud turned down; 2200 Hz is "bite" or "claw," but you can turn it down if it sounds too aggressive; and 6600 Hz is "crisp" or "sizzle," which I tend to leave right in the middle.
And then, with your GEQ basically figured out, you can tweak the tone stack again to get the sort of distortion texture you want. Don't worry if you find yourself changing things every time you play for a little while: it's very easy to get EQ ear fatigue if you spend too long bringing things up and down, so (and this is important) after you get your basic TMB and GEQ settings, change them in small increments, and give your ears time to adjust.
For Crunch mode, first of all, I should say that I never use it with the GEQ, so I can't recommend GEQ settings that will work for both it and channel 2. As its standard controls go, I find that Gain and Mid set lower (and remember, the second half of Mid is also your Boost on ch.1) get you closer to brownface Deluxe/Suhr Hombre/ZZ Top territory. The more you push the Gain and Mid/Boost, the more you get into hotrodded Marshall sounds; if you push that Mid/Boost to, say, 3 o'clock, and then slowly turn up the Gain from about 9 o'clock, you'll start off in JTM range, then up through Plexi, modded Plexi, 2203, and something close to Soldano and Bogner at the top end.
Since I don't use the GEQ here, it's simpler to dial in: just choose the amount of gain and character you want with the Gain and Mid/Boost controls, then dial in Treble, Bass, and Presence. I keep Treble around noon, I always turn Bass down as Gain goes up (that's a good rule for any amp, especially a Mesa/Boogie), and I don't like super sibilant, crispy tones, so I usually leave Presence around 9:00-10:00 on the Crunch mode. If you wanted a more accurate Super Lead or José mod Marshall tone, you might want to run Presence a little harder. Even at low gain in Crunch mode, I doubt you'll ever need Bass higher than 11 o'clock or so.
