I have a Mesa Boogie Fillmore 25. The clean tone is great. The low and high drive settings are lacking depth. I think they're more for metal. I called Mesa and inquired about their tubes and the technician told me I can use any tube I like and he encouraged me to experiment which was a surprise to me because I thought my warranty would be dishonored by using other tube brands, but he said it would not and again encouraged me to experiment. I have a matched/balanced set of Tungsols that I bought for another amp but never used. I put it in V1 and wow, it made a huge difference in not only the low and high drive settings but also the overall sound. I dug the sound before but the Tungsol in V1 gave the overall sound more fullness. I was totally satisfied but this morning I figured if that one change made that big of a difference maybe I should really experiment. I proceeded to switch the V2 slot and wow, the tone had even more body and fullness. I couldn't believe my ears and couldn't stop playing. Now I'm able to used the second channel strictly for overdrive without pedals. After about 2 hours I decided to put my third Tungsol in the V3 slot. The overall result made the tone slightly brighter taking away the fullness. However, I decided to try the Mesa tube in the V1 slot leaving the Tungsols in V2 & V3. This resulted in a far richer tone than any other configuration. I'm leaving it as is. Mesa in V1, Tungsols in V2 and V3. I'm not going to mess with the V4 and V5 spots as they are loop and reverb stages and I don't use the amps loop or reverb.