6550 tube vs KT88

Can't speak to the KT90's, have not heard them.
A while back, I was auditioning an Audio Aero amp. It could take either 6550's which worked in my Conrad Johnson Premier 11a or KT88's which came with it. My experience with my system showed the KT88's to be a more bass heavy and just not enough treble magic when compared to 6550's in that amp. Having said that the KT88's gave a great deal more detail than the 6550's but did not sing for a violin or have the clarity of a triangle or piccolo for my ears.
It was always a case of did I want more resolution of the singing voice. Hope this helps

Yes my limited experience of the 88's showed not much resolution for classical. Too smooth. Smooth being if you took the fq range as looking overa falt landscpe, you'd see few peaks and valleys. Bland IOW.
The 90's in my Jadis present descent bass/mids/highs and all 3 are separated, not mushed together like a 88 tube does. All 3 in equal balance. I'd say the highs are slightly limited with the 90's, but make up in a musical image.
I notice there are very few amps offering 90's.
I don't think I;d like the 6550's. seems got lucky, the 90 may be the ideal tube for my taste, offering dynamics.

I found zero bass with the 2 88 tubes I tested. Just a falt response, flat like the Kansas landscape.
I can see how tubes get a bad reputation vs a big ss amp.