Chaps, you can't willy-nilly swap tubes of the same pinout: Filament currents vary from 1.4A for the KT77 to 1.7A+ for the KT150.
Power is calculated from B+ and the current available. Changing from KT77 to KT88 will not provide more power. It will sound different due to the transfer function of the drive vs the tube power curve non-linearity. Along with the mismatch inherent in ALL tubes of the same type.
IGNORE ALL recommendations for a specific tube from Joe AudioPhool unless he has the same equipment in the same room! AND listens to the same program.
Myself and another really like the JJ KT88 in our M-125 monoblocks. He has Klipsch and I have custom ET LFT8b. I've never heard his system and he's not heard mine.
Apex Matching is really good.
See ieLogical Rolling for more on matching and aging.