I've been loving the different wattage levels on the MKV. Like others have said regarding channel 1, to get that real clean tone I need to have it on 90w. I'm going to keep fussing about with 45w (diode) on channel 1 and see what happens, but I'm constantly finding even with the gain at 9:00 there's still some breakup. Not much, but it's there. Could be one of my speakers, however. I've got it on Fat mode with the Bold switch engaged, and when I'm playing through the neck pickup it sounds great. Then again, I only play through my neck pickup..
Channel 2, however, is where the real magic lies for me. I can get down with Edge and Crunch, but Mark I mode is the winner. With the gain at 10:00 or so and some fine EQ tweaking I can get some nice desert influenced guitar tones. It seems to really favour 45w (tube), and when I drive it with a Micro Amp it opens up very nicely. 90w is just more of a good thing it seems, and I love the way it breathes but for some strange reason I just like 45w better. Great for doing some louder riffing or leads.
Channel 3 is still a strange one for me, but I've gotten some interesting sounds out of all three wattage settings. I don't do much metal besides a little High On Fire or Tool here or there, but the Mark IV mode seems to deliver just enough bite, however comparing it to some of the other channels I find the amount of low end is slightly lacking. Haven't tried dialing in Extreme too much but that might be my next stop. Pentode is my preference, though I still want to explore Triode a bit more. Just a side note, all my experiments so far have been on full power. I've yet to give Variac and long listen.
I'm still only a couple months into owning this amp, but I can say as a newcomer the most satisfying part has been experimenting. Finding a good part of the tone that I've been chasing has been rewarding enough, but the variety of tones that I have at my fingertips on top of that is like having my cake and eating it too.