Re: Mesa amp distortion: Mark, Rectifier...
To the original poster:
I know that one of the big selling points of the Mark series amps (form the Mark II onward) was that they incorporated Mesa's "Simul-class" technology, in which one set of tubes runs in Class A, and the other set in A/B, which is why the wattage numbers are slightly lower than a standard all A/B amp. This is supposed to give you an overall richer, sweeter tone, and in my experience it works. This is also found in the Simul2:90 power amp.
I will say that I always had a hard time dialing in sounds on the Mark series amps. I can grap a Recto or Tremoverb and get good tone in seconds, but the Marks always gave me trouble. Way back when I worked at GC, I asked Trent our Mesa rep (a really cool guy), and he was able to make the MarkIV we had sound simply AMAZING. He explained a lot of the particulars about the Mark amps that I have now forgotten, but I remember that he said that a lot of people have problems with the Mark amps because they are designed differently than your standard Marshall/Fender/whatever.
On the newer ones, there is the standard Bass/Mid/Treble, and also a graphic EQ. If I remember what I was told correctly, the Bass/Mid/Treble controls are all pre-gain EQ, whereas most other amps apply gain first before the EQ controls. This apparently changes how the controls react. He said if you try to dial in the basic "V" that most players use, it won't sound right. He set the controls in what appeared to me to be a very strange manner, with the bass very low, high mids, and so forth, adjusted the gain, and then set the graphic EQ, which is applied after the gain, to a sharp "V." This resulted in absolutely killer liquidy gain. I was amazed at how great the amp sounded.
I don't know about the older amps, but newer Mesas come with a ton of "sample setting" diagrams, and you can always call Mesa and ask their advice. I've done it before and so have friends of mine, always with great results.