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
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