proaudiostar on ebay has the beyerdynamic T90 open box UNUSED for $409 FS. they're an authorized dealer when i looked them up on the site.
this is the message i received back when i asked about what they mean by open box UNUSED.
"Thank you for contacting Proaudiostar.com We have both (NEW) and NEW other (Store display...
i've been thinking about getting a 6F8G to 6sn7 adapter and buying some 6F8G tubes. there should be no problems since they're the same from what i've read, right? this is the adapter i plan to buy
thanks spacequeen7. i sent them an e-mail asking for a 5998 replacement with higher test ratings and used this thread (and your post) as a reference in my e-mail. now i wait to see how they will reply tomorrow. i mean their site says they store NOS and factory-new tubes. i knew the test results looked concerning and your post only confirmed my suspicion.
both of those links don't mention a TS black glass 6sn7gt square plate. the second one doesn't even review a black glass at all. the reference thread only mentions a round and oval plate. more specifically on how the round plate sounds. i've looked through it before.