I modded mine with some 12ax7 Fender Groove Tubes I had lying around and I found an improvement. As you will read, if you do your research, where the tubes are placed in the MPA II's schematic, they really won't change your sound all that much. I'm pretty sure anyone who says "oh absolutely must use THESE tubes, they're warm and juicy" or "oh I use THESE tubes, they're fat and chimey", etc, etc are all cases of "The Power of Suggestion", for this mod anyway. The only real sonic difference I really heard was that it helped rid me of some brittleness I was experiencing when louder notes were sang on vocals. The tracks were never clipping and I wasn't overdriving the gain,. but at times I'd get this thin, sharp, fizzy sound in the high end, likely due to cheap Chinese parts, especially the tubes. I found it is a lot smoother sounding now. I think the rule of thumb here is, ANYTHING will sound "better" than the cheap Chinese tubes that ship with the unit. It's an easy mod and I'm sure they almost expect you to do it.
Audiofire12 | PT10/11 | iMac | 2xBCF2000 | Big Knob | NS-10M | BX5a Dlx | KRK10S | Auratone | JBL J2080 | x2 Pre73 MKii | M80 | Pro MPA II | 7720 Comp | Pro VLA II | dbx163X