I have had 2 heads modded, one by Trace's company and one by JerryP at FJAMods. I have had similar changes done to both amps and it really took both amps to a level not attainable by just replacing tubes and/or speakers/cabinets.
The first amp I had modded was a Peavey Triple XXX.
I was playing a Mark IIC+ and wasn't happy with the tone of the clean when I had it set for crunch lead sounds and didn't like the crunch sounds when I had a good clean sound dialed in. I decided that I wanted to buy a high gain three channel amp that had separate EQ and gains. I had had a Dual Rectifier when they first came out, a 2 channel, and absolutely hated the clean channel so I didn't play one during my testing, my mistake. I tested a lot of amps before buying the XXX. I played the JSX and 5150II and thought that for what I was looking for the XXX was a better fit. The 5150II didn't sound good for clean and the JSX didn't get there for the high gain sound. They were all around a grand at the time and that was a little easier to swallow than the $2000-4000 for some of the better quality amps. I was also interested in the Splawn, Voodoo Amps, Framus, Bogner and VHT models but they just were too expensive, or so I thought at the time.
After I had owned the Peavey for a few months a friend of mine got me started with modding effects pedals. I bought a couple of DIY kits and really liked the results of the pedals. I really liked what it did to driving the Mark IIC+ into singing leads, but it took some volume to get it there. It was then I started reading about modding my amp and what everyone that was doing had to say. It sounded EXACTLY like what I was seeing with the effects pedal mods so I pulled trigger and sent my XXX to JerryP. I was VERY please with the results.
I still wanted an amp that sounded great clean and the XXX was better with the mods but not great. And I wanted a clean channel that I could have sparkling clean or 'pushed' clean and this meant I needed an amp with a preamp gain control on that channel, the XXX didn't have that.
I sold the Mark IIC+ and used the money to buy a Lonestar that I played at the dealer that I thought was the Best sounded clean I had ever heard. When I was a kid my dad had an original 65 Fender Twin so I knew what I wanted and the Lonestar was it...
While I was playing REALLY loud one day I had a power tube short out and arced one of the PC boards in the Peavey and I had to get it fixed. This took about 3 weeks due to a backlog of work the tech had. So, in the mean time I got to thinking that if my XXX sounded this good, what would an amp that was better built, component-wise, with a closer match to the feature set that I wanted could sound like.
So, I went to play a VHT Deliverance and really liked the way that you back the guitar volume down and the sound would clean up. I couldn't do this with the Peavey. I then went to play 3 channel Dual Rec and it could do the same thing but wasn't quite as clear sounding as the VHT. This is very hard to describe and no amount of Online Clips can represent this but the sound quality was much better. I really liked the DR but I knew from the start that it could be better so I bought it with the intent of getting the clarity mods done and having a Mercury Magnetics output transformer installed.
The result was dramatic, to say the least. From first impression you could be mistaken to believe that there is less gain but what really is going on is the gain is the same but the 'noise' is gone. The hiss, fizz, buzz, crap, cheap-*** poor quality stuff is gone and what is left is pure and musical.
Read the Harmony Central posts from others that have done this to their 5150's or DR's or XXX and remember that most the owners don't post after having this done.
There are people that want the high end sizzle/fizz and so for some this mod is not going to be worth it. And there are those that try making an amp that they don't like into something that is not possible to achieve. If you want a Marshall sound, buy a Marshall. If Fender is for you, same answer. A Mesa is a Mesa, perios. You can clean up some of the sound by swapping preamp and/or power amp tubes, getting different speakers/cabs. But imagine for just a moment if you started out with an amp that the tone and gain was more pristine to start with and then you swapped the tubes and speakers around to get EXACTLY the sound you want.
It use to be that Mesa would do these kinds of mods for you, for a price. That's why there are so many different Mark versions, they would modify something for a certain player and realize that this was a good thing and changed production to accommodate this change into the line. They've gotten too big for that now and so you have to look elsewhere. The Dual Rec is a good amp that only needs a little tweaking to make it a great amp. I think you could get almost there by just replacing the OT with a Mercury Magnetics. If Mesa offered this as a factory option, people would buy it once they heard the difference. It made my Peavey sound close to a VHT, for God's sake!!!!
Like I said, this is a great mod if you already like the sound of the Dual Rec but want to clean up the sound just a little.
Try to remember this: the sound your amp makes is only a small part of what is important in being a performer. You still have to play well, fit in with the band, balance your volume and have dynamics. In a live situation running through a loud PA with a good soundman I doubt anyone in the audience would know if you had a $4000 Herbert or a $600 DigiTech so don't stress about it. We make the magic with our hands, not the amp.
I spent about $2500 getting my Mesa, (*$1800 for the amp, $700 for the mods and shipping), where I wanted it to be and would not be able to recoup that cost if I changed my mind so be SURE that you want to keep the amp before doing this or even considering it. A new VHT UltraLead is $3200, a Framus Cobra $2200, you copy?
Hope that helps.