I used NOS octets of Sylvania 6550s for a few years, purchased from Upscale, until they ran out of stock. Kevin Deal recommended the Winged ’C’, Svetlanas, as his number one choice for KT88/6550 types, and they were close enough in performance to satisfy. When the Svetlana plant in St Petersburg burned (2012), production of that (very nice sounding and reliable) tube ceased forever. I then found a supplier for octets of NOS GE 6550As, which were superior in every way. My new favorite 6550/KT88 tube is the Gold Lion KT88 repop, from Russia. A conversation with either Kevin Deal or Brent Jessee would be invaluable, far as the straight scoop on the sound and reliability of the current, "best", far as new production KT88/6550 types and what became of Svetlana’s version. Some history, regarding the Svetlana plant and 6550 (Winged ’C’), here:
https://valvetubeguitaramps.com/svetlana-winged-c-valves/ Lots of opinions, here (note the dates):
https://db.audioasylum.com/cgi/search.mpl?searchtext=winged+c+6550&SelectForumtubes=tubes Of course; everyone’s ears and systems vary. A couple tube purveyor observations:
https://www.thetubestore.com/winged-c-sed-6550c and (bottom of page):
https://www.tubesforamps.com/6550-tube-selection-guide