I threw some KT-66s in the power section and it helped smooth out the semi-grainy mid range I got out of Channel 3. The 6L6s weren't bad, but I had some extra KT-88s on hand and they made a significant difference to my ears, even the 6L6/KT66 combo is pretty cool. Also a cool trick if you don't want to bypass the loop and have it unbelievably loud is to activate the loop and back the signal down a little, this allows you to crank the output volume on the front of the amp and it creates a much thicker/more full tone. I think Petrucci suggested this "trick" and I find I use it religiously, I love extreme mode on channel 3 in 90watt mode. Also using Bare Knuckle Nailbombs help a lot, I've played many other pickups like Dimarzio D-Activators, various SDs, and an EMG 81 (the 85 sounded good) that I couldn't stand with the V next to the BKs. The Mark V is an amp that you have to spend time with to reap the benefits, this is the main reason people sell them. I have had mine for over a year and I have no regrets, it's an amazing head with a lot to offer. There are still a lot of amps I would like to own or try out (*cough Royal Atlantic cough*), but I don't see myself letting it go, it's that good.