cellardweller and everybody,
A tone control can only govern so many frequencies at once, and is attenuated to a center frequency. As an example of this, when Randall came up with the Stilletto series, one of the advertising points of that amp was that its bass control governed a higher range of bass frequencies than that of the Rectifier series, since the bass response of a Marshall (which is what the Stilletto is Mesa's answer for) isn't as low as the Rectifier.
Let's say for example that the main "oomph" of the bass frequencies on a Marshall was around 250 Hz. Setting the bass knob on a Marshall to govern 80 Hz isn't going to do very much, since that amp doesn't really reproduce those low frequencies in any great quantity. Setting it at 250 Hz would give you maximum control over the bass frequency range that the amp produces the most of, thereby giving you the most control over the overall amount of bass. This is a logical thing to do, and what the manufacturers indeed practice most of the time.
As another example, let's say you owned a Rectifier and liked the tone overall, especially the great amount of bass it puts out. Let's also say that you prefer to play with the tube rectifier rather than the silicon diode. However, you'd like to tighten up the bass a bit and get rid of some of the mud/flub that happens when you crank the bass knob, but without having to dial your bass knob back so much that you lose all the bass, or without having to switch to silicon diode rectification.
The answer might be to put a good graphic EQ in the effects loop, where you could leave every slider on the EQ at "0" except that you carefully lower the slider at 200 Hz until just the mud and flub go away, but the rest of your bass is still there. Or you might actually like the mud and want even more of it. Or you might want more percussive thump from your bass response for heavier chugging, so raise the slider at 100 Hz just a bit.
This applies to mid and treble frequencies as well. Let's say you like your Rectifier's tone overall, but you're playing some old Metallica. Rather than scoop out all the mids by turning the Mid knob way down, you could just put a graphic EQ in the loop (not, I repeat, NOT in front of the amp) and turn down 800 Hz a good amount, and maybe 1.2 KHz a tiny bit. This is the classic mid scoop and will get you mastering your puppet in no time.
Or, let's say you play an Ibanez RG through a Recto in a cover band, and your song set list has some Foo Fighters, but it also has Journey's "Any Way You Want It". Since the Foos recorded with a Recto, getting that tone is effortless. However, nailing the Journey tone is going to be a bit tougher on a Recto, compared to a Marshall or a Stilletto. Never fear, just leave channel 3 to do the Foos, and set channel 2 to Vintage, with tube rectification on both. You could keep a graphic EQ in the loop set to cut 200 Hz mud, slightly boost 400 Hz Les Paul "woodiness", and significantly boost 800 Hz for that mid-heavy, singing tone of the 70's. When it came time to do the Journey song, just change to channel 2 and kick on the EQ and there you are. No need to go and buy a separate Marshall and Les Paul.
Graphic EQ's are all about giving your amp maximum tonal versatility. They give you more precise control over your tone compared to the relative hamhandedness of a typical tone knob. Some folks honestly can do just fine without one, especially through a great amp like a Mesa. However, the tweakers are going to love the extra precise control they'll have in nailing a particular tone, and let's face it, the tweakers are the guys who care enough about their tone to end up buying a Mesa in the first place rather than a $100 Wal-Mart special or even a crappy recent Marshall. :mrgreen:
