The amp is nearly 30 years old. If it hasn't had a cap job it certainly needs one. When caps are failing they can cause all kinds of weird stuff. Things like you describe, strong initial attack with the notes just going away & not ringing out. Also a pronounced loss
of bass response (amp sounds brittle). Unexplained distortion. Most importantly should a large filter cap short internally (to ground) it can take the power section with it. (tubes, resistors and the power transformer as well) all before the fuse blows. While I've never personally seen it happen to a Mesa poroduct, I have seen it happen to Musicman amp (was playing at the time) and I have seen an old Fender Bassman that was damaged in this way (with original caps in it and one of the large filter caps WAS shorted to ground and BURNED as well)
I sent my MK2B to Mike Bentinelli at Mesa for the cap job as soon as I acquired it along with the effects loop mod. It sounds awesome, I couldn't be happier. People hear this little amp and just shake their heads (it's certainly not my playin'). I've been told it's the best sounding amp I own by several friends who've played it.
That'ld be my guess, anyway if you've swapped tubes you've done about all you can. Probably be a good idea to have it checked out before something really breaks.
