Hi mike, and a belated welcome to Euforia land. And you have indeed struck gold with tr's AE model, complete with what I personally also found to be the best (until now) performing tubes (in my system!) of the many 'alternatives' I've experimented with these past few years - the KT88...firstly in this case the Russian reissue Genalex Gold Lions that you have, and then the much rarer and more expensive original GECs (mine being the top anode TT21/22 versions). As with most original 'NOS' tubes, they do perform a good bit better than later reissues, but prices now can be way OTT unfortunately.
However, as
@connieflyer said, the Russian military GU50 is proving to be an exceptional tube, and especially given its ludicrously cheap price - on average $4 to $5 each!! (plus shipping). There are numerous East European and Russian ebay sellers who bought up vast (over)stocks from the Russian military, and these
Russian tubes are generally regarded as superior to the Chinese
FU50. And it would appear that uniformity of production is such that it matters not if produced in the 70s or 80s...they're all extremely well built tubes lol!
The only trouble is at the moment there aren't any 3rd party adapters that are needed - they should have been on line by now (from China) but everything there is up in the air of course. Another source could possibly be member Deyan in Bulgaria, if anyone is interested in contacting him, but don't know if he'd be prepared to make this particular adapter or how much it would be. I myself have made just a few, but I'm afraid I can no longer make them generally available alas...
As for the 'dual adapters'... having discovered that combining a GU50 with, in my case, a GEC TT21 version of the KT88 per power channel resulted in such a leap in performance that I hoped previously available dual 6SN7/6AS7G adapters might help folks be able to try this pairing using their reissue KT88s. However, they can prove unreliable and so I myself no longer recommend using them...which I find a real tragedy, but something experienced DIYers could possibly gain much benefit from tackling.
As
@Scutey mentioned, there's a vast array of tube (and other) recommendations throughout this and Elise threads...if you can spare the time to gradually sift through... my own (tube) posts have been exclusively on 'alternatives' to those configured for these past years, if you're interested. But as also previously mentioned, take your time...this can indeed become a bottomless pit (that many of us have fallen into lol!). But you already have both an amp and tubes that would satisfy most keen hobbyists...all will depend upon your level of curiosity...and/or depth of your wallet!!
Whatever, I wish you - and everyone - HAPPY LISTENING!...CJ