KT88's DO sound different; it's hard to explain...fuller, more musical. If you can bite the bullet, go ahead and try them. KT90's are a whole different story. They are fantastic in Sunn guitar amps, giving an EL34-like break-up, as opposed to the glassy brittle sound that 6550/KT88's give to six-strings. In Sunn bass amps, they tend to mush out too much; that's why 6550/KT88's are used as they have mucho headroom, don't break up until really pushed, and are pretty powerful. Count on replacing most of the capacitors; on a 30+ year old amp, it's just a matter of time before most, if not all, of them dry up.