sounds to me like the culprit is your tubes and who ever installed them probably didn't bias them as hot as before..
hotter = fat and bloomy sounding.
colder = more agressive and thinner.
try stick with the original brand of tubes and find out what kind of bias your ears like, I like about 60% dissapation on my marshalls which is quite hot but not as hot as most who go as high as 70% which is the limit.
As for bkps, i'll leave everyone else to suggest that.