6550 tube vs KT88

Was wondering what might be some of the differences between these 2 tubes. I've heard 2 sets of 88's and was not a tube that fits my music, classical. i found the sound too compressed, too flat on fq's, dynamics were thin. Too warm, not detailed like the KT90 tube.
Is the 6550 a middle of the road between the 2?
Hows the timber? bass/mids/high separation?
The KT88 seems to blend the 3 too much.