I experimented with Ei KT90v.3 a while ago with my then owned VAC PA100/100. Biasing problems frustrated me enough to permanantly distrust them. Now, over a year with my replacement to the VAC amp, I too appreciate the mix n' match possibilities with my Audio Valve Challenger monoblocks. They were delivered with Sventlana EL34's then I ordered (12) JJ KT88's. Fat bass became solid bottom end, a musical, sweet midrange connects the bass to extended high frequency clarity. The versatility of the amps allowed me to mix (4) KT88's w/ (2) EL34's in each amp. I haven't tried 6550's due to my satisfaction at this stage and $$ for yet another dozen of tubes.
