6550 vs KT88 vs KT90


From what I hear, these tubes are interchangable on most amps it seems that the 6550 is the least loved tube of the bunch. What are the sonic differences of these tubes? Thanks in advance for your reply.
jman
I second the vote for the Svetlana KT 88 vs 6550

It is exactly the same tube,, one can not win out over the other.
If anyone believes they are dif, its snakeoil.
Resurrected an 18 year dead thread, so you can respond (to yourself, repeatedly) about everything sounding the same?       The only thing, "the same" about the Svetlana 6550, that’s been made by Sovtek since SED’s St Petersburg (Winged =C=) plant burned down and took their manufacturing equipment with it (some years after this thread started/ended); is the name that Sovtek/New Sensor/Reflektor acquired.       Not saying it’s a bad tube.            Simply not the same tube.       Just as those that are now manufacturing, Mullard, Tung-Sol, Gold Lion/Genelex, etc, repops.      None of them are the same, or- sound the same, as the original valves.       I have noticed; some of the most reputable tube mongers that used to sell and and extol the virtues of the SED/Winged =C= brand tubes, aren’t selling the Sovtek repops.       Doug’s tubes does list Sovtek’s Svetlana 6550C as, "A great choice for people on a budget."        To each his own, of course!

Well I actually sold the original NOS /very low hrs, maybe 200 hrs, on the dozen sold  for $250,, and bought the KT90's, which I then(10 hours useage)  sold, bought 150s(10 hrs usage), returned, rebought 90's(10 hrs) returned, bought 6550s(10 hrs), returned, rebought 6550's and thats what I am running. 
Svet 6550's
which according to the Ebay seller's site specifically states,
The new Production Svet  6550 are *EXACTLY SPECIFICALLY PERFECTLY IDENTICAL/NO DIFFERENCE to the  NOS Svet 6550s*
I believe him
Why would he lie about it?
If you take the print off a  NOS and new prod svet 6550, could you tell them apart?
Of course not. 

You’re talking about an eBay seller:      "Why would he lie about it?"      Hilarious!        Provide a quad of unmarked, NOS original SEDs and the same, of the later Sovtek version, to install in my monoblocks (a quad each) and bring your wallet.      I’ve never been a big gambler, unless the odds and experience are in my favor.      btw: You couldn’t have heard the full potential of any of the new tubes you returned, after only 10 hours.