6550 vs KT88 vs KT90

I love the treble and midrange of the EL-34 but find the bass unacceptably fat. I love the bass of the 6550C but not much more. Trelja, how does the bass and midrange of the KT-88 or KT-90 compare with the 6550C?

I've learned a great deal from this thread and I have been having a great time tube rolling. My Audio Valve Challenger amps actually allow me to use ANY combination of 6550, KT88, KT90, or EL34 tubes so I have been mixing and matching to fine tune the sound (and generally driving myself crazy). I have not tried KT90s partly because of the expense but also because I am told they will only work optimally in an amp with appropriately designed transformers. Any thoughts?

Many thanks to all on this thread and especially to Trelja who REALLY knows his tubes. This is what all the threads should be like. Mark me down as another happy KT88 owner. Anyone for driver tubes?