Thanks. So, lemme get this straight . . .
From most "AGGRESSIVE" to smoothest:
Mark IIC+: The benchmark "aggressive" amp.
Mark III: More aggressive than the Mark IV.
Mark IV: Smoothest, tightest gain.
By "aggressive" I interpret that to mean a more throaty, grainy distortion, full of mids, yes? I finally dialed in the the most aggressive tone on my DC-3 last night using my Les Paul Studio (which pwns my DiMarzio-equipped Strat). Previous to this, I was getting very boxey, flat recordings (too much bass). Will post a clip as soon as I can.
I really like my DC-3 now that I've got it dialed in, but I'm still gunning for a Mark IV. It's possible that I really want the sound of a Mark III or the famed Mark IIC+. But I really wanted a "modern" amp that would contrast well with my DC-3 and my 6505. Or, one that simply does what my DC-3 does, only much, much "better."
I guess "nu-metal" is what I'm looking for in the Mark IV. I don't necessarily want it all the time, but if I want it, I want to know that the amp can pull it off. But I also really like those "barky" mids that make up that signature Mesa growl. The new Mark IVs I've demoed recently sounded tight and "razor-ey." Not as dull as my DC-3. But as I said, I finally dialed in a very good metal sound last night on my DC-3 with a lot of "meaty" aggressive tonal characteristics. When I get a chance to post a clip, you should get what I mean. Any further suggestions, recommendations, comments?