KT-90's have a little more power than KT-88's or 6550's. They might have a little (very little) tighter base, but do not have the richness in the midrange that the other tubes have, IMO. I have a pair of MFA-M200 mono amps and have rolled KT-88's and various kinds of 6550's. I recently put in RAM 6550's which are matched by RAM Labs and are actually Sovtek 6550 WD's. These are the best tubes that I have tried on these amps.
