I always get confused and know others are confused when talking about Celestions lame use of model numbers for their speakers. Too many G12 types is the culprit for all of the confusion. I then throw in the the word greenback and it's no wonder. G12-30, G12-100, G12-65, G12K, G12T, G12H Heritage, G12M Heritage, G12 Anniversary, etc. etc. etc. - even the blue and golds have G12 on their labels...every 12" speaker says G12 on it somewhere...stupid Celestion! Whew, glad I got that out of me

That said, I like their speakers mostly. I recently dumped a true V30 out of a JCM800 single combo build after A/B various speakers of which the G12 Heritage was one of them and it was clearer w/ more bottom AND top end but didn't break up as well in the higher gain territory. The G12 sounds fantastic in a 5E3-ish type of build more in cleaner settings and I don't like them for heavy gain type of amps. I ended up going w/ an Eminence Wizard which is one of the best speakers out there all around. Clear, balanced, efficient, sweat, pretty much all of the adjectives you want to attribute for a good sounding speaker.
*I think the amp sounding a certain way "muddy &/or boomish" should be looked at in the tone control section and it's use of coupling caps and/or voltage dividers or bypass caps and such also.