6550 vs KT88 vs KT90

Rcprince, I am happy you put the question to Jadis. As stated before, they really seem to love the KT90. They go out of their way to recommend it in their products. I think Frank Garbie is a great guy, and seems to offer a lot of help. I have however, gotten information from him that conflicts with what Jadis advertises or says. Particularly in tube substitution. He wasn't aware my Orchestra Reference or some other Jadis amps accept tubes other than what they came with. He has given me excellent help when I have gone to him. I know horror stories abound, but I have nothing but positive things to say. I am sorry but I can not offer anything in regards to old KT88s, as I have no experience with them. I apologize. Enjoy the tubes, Krys!

I am trying to replace my tubes that originally was KT90's on my Jadis orchestra reference integrated amplifier. Beside that the KT90's are quite good I am curious of experiencing the KT88's specially from Valve Art, hoping to have little warmer sound with still good bass and extended treble of the 90's. If you have any experinence It would be great help. Thanks

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.

Krys, I found the KT90 to have the best bass of the three, far and away.

Yeah but these Svet 6550's with Defy7 powering/new M Supreme SG caps, + Seas Thors new Munorf xovers  = bass like a  sub amp, So 6550's offer tons of bass.
But also, from what i ;'ve read on disc boards, beats out the 90 in terms of wider, more open sound stage. 
No i have to side with the 6550(=kt88, exact same tube) vs the 90, which may as you say be more clinical, detailed (less warm, warm to me means more mellow the reverse of warm is more attacking, more forward,,which if i recall the EIkt90's in my Jadis Orch Refer were more forward, attacking,,and,,harsh. The EH kt90 i am sure does not suffer from this harsh weakness as did the wayyy over rated EIKT90/  Glad that tube is now extinct. 
No I have to give the edge to the 6550/88 (same exact tube) over the nice, but no cigar,,KT90.

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.

Don't need to roll to 6550's,
Your KT88's are in fact none other than,,,yep you guessed it, due to miniscle differences, same as mt Svet 6550's ,
IMHO the Svets6550's.  will shootout all other 88/6550's ever made.
In all categories,,including the most important characteristic, reliability. 
Built like tanks, sings like angels...