6550 vs KT88 vs KT90

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...

I am trying to replace my tubes that originally was KT90's on my Jadis orchestra

East EU tubes are like chinese power tubes, unrelaible, Those KT90EI's that came with the Jadis Orch Refer went bad after 1500 hours. = Junk  and  set the record for screechy harsh sounding. 
They fetch like $100 each now,,lol...
But when new, set the record for life like sound stage/female voice. WOW,, but hard to deal with the  steely highs. , The Svet 6550 has none of these issues. 
Its a tube you can <<relay on>> session after session,,and on and on, for years, = faithful tube. =  Gold winner.

experience on how the new KT88s stack up against the older ones, sonically

NOS Svet 6550(kt88) is ientical to the new production,, so don't pay high prices for *NOS Svet 6550s** Its snakeoil. 
No have not tried EH KT88's, vs Svet 6550's. 
Defy takes 12 tubes = not practical,,I did hear the Gold Lion's,,and found them to be a  bit lower vs the Svet's,. so i returned to vivatubes/12 GL KT88's,  with a  $150 restock generosity fee , and also previous 2 weeks, returned  vivatubes kt90's $100 restock gift fee,,,they said * please sir, go do business elsewhere*,,so I did and rebought for a  <<yep>>a   2nd time,, the Svet 6550's from a  California seller, and that is the end of my long  very expensive search for a  good power tube.