Svetlana KT-88

I would look at the KT120. It's far superior sonically in both of my KT88 and 6550 amps.

The dealer means that my amps trafos can not handle the KT120.
Is a Unison Research Performance.

KT-88's are relatively difficult to manufacture and there are a lot of mediocre to bad ones being sold - people with amps that use them must be very careful. E.A.T. makes high quality KT-88's, but they are expensive (the list price for an octet was $1,400, the last time I looked).

Ghosthouse is correct: current Winged C production comes out of St. Petersburg.

I would contact Andy Bowman at Vintage Tube Services in Michigan - he is an excellent tube vendor.

the right equipment and it is worth noting that very positive things are being said about the Gold Lion reissues, in particular (esp. their KT88s)

^^^^
Yes the GL 88/6550  are popular and have accolades.
I personally found the Svet's a  better sonic balance and voicing in my Defy.(I returned the GL witha  restock fee) Also read another audiokarma comment pretty much in agreement with my listening to the GL's.
The ebay seller posts on his site, that the NOS 6550  Svet are identical to the new prod 6550
I am not a believer in this idea of winged C , SED thing.

Also note on TAD's web site, their top of line 6550 @ $100 are in fact from the Svet lab = New Sensor.
So a  NOS Svet <wing C/SED > = New Prod/New Sensor Svet 6550
Both with  have idential performance, as it is the very same exact tube.
Best site for exactly matched Svet 6550's is Amplified Parts, pay a bit more, but  receive exact match,, which for this highly sens Defy 7, <<only 1 bias UNDER CHASSIS/= Need Tech to bias>>>  perfect matched 4 sets of trios is critical.
These 6550's will perform 2500 hours, 
So lets say you use your amp 4 hours a day (I listen at least 6 hrs) X 350 days = 1400,
 so every 18 months you need to bring in new power tubes. 
I'm going to stick with Svet 6550's, as they have a  nice tonal balance and are clear, articulate, bass rich, vocal reproduction  <,stage like>.
+ they image Allan Pettersson's 8th sym BIS with emotional involvement = testing reference passes with high ratings. 

Just some general info: There are no SED, Winged =C=, Svetlana tubes, currently in production.     I really liked them (bought from Upscale), but- the St Petersburg plant burned down, years ago (along with it’s equipment) and SED closed the doors.     New Sensor bought the name (ONLY) and is manufacturing their version in the Sarartov, Russia plant.      Some info from a reputable dealer:  https://www.thetubestore.com/winged-c-sed-6550c                Try contacting known to be honest tube mongers, like the above, Brent Jessee Recording, TC Tubes or Upscale Audio, for knowledgeable info, as to whether they are, "the very same exact tube."      An opinion/observation regarding the new version:  https://www.tubesandmore.com/comment/2577/report                                   Compare the plate and getter construction, to the above (original Svetlana 6550) pic, here:                   https://www.ebay.com/itm/PLATINUM-Matched-Quad-4-Svetlana-6550C-tubes-Russia/184570896793?hash=item2...