How expensive are best NOS tubes ? And how difficult to find ?

Well for me the TS 6550 bp has the same glorious mids as the EL34's but adds a hair of warmth and has the lows EL34's don't have besides more power. I felt the EAT KT88 has the most linear sound from top to bottom along with the original KR Audio KT88's. Just saw an ad for new KR KT88's which may be back in production. Ei KT90's are somewhat linear with a bit more in the lows but had the best sound stage of them all. The original Gec KT88's are also quite linear.I didn't use them much since I preferred the Ei KT90.
I'm sure transistors also have a sound signature too. To me Pass labs or Dartzeel is a much warmer sound than SimAudio or Soulution as a couple of examples.
Going back to speakers I preferred a warmer sounding amp when I had the Dyn C1's and a more linear sound with Raidho. But there is also the beauty of a tube amp. If you want a different sound signature you can swap out some tubes and not have to purchase a new amp.

My system has been basically the same for more than 25 years except for the CD player.  ARC pre, LS2B MKII + remote takes a 6DJ8 but has been on a diet of late 1960's to early 1970's 6922 Cca Siemens and Halske grey plate NOS for the last 20 years. Did a big shoot out 20 years ago and except for one stint with a Valvo pinched waist 6922 Cca from 1958 it's been all Siemens.   The problem is 20 years ago they were plentiful as brand new never used @ $80 a tube and now they are $400 or more for a tube that still meets NOS readings.  There are no new ones I can find so fortunately I will live with the three new ones I still have in stock before trying out some Russian Rockets from the 1970's that are new and were highly touted.  AFAIC, the best are long gone.

I too compared 6922’s 20 years ago, and I found that the best tubes were also the best bargains for what they were: HP branded. Back in the day, stories circulated that engineering students got part time work at HP testing tubes - and rejecting 95%. Let that sink in - rejecting 95% of NIB premium military tubes.

Guess that’s why the moon shots came off.

The best 6922’s were pinched waist Amperex until I discovered HP selected tubes, and the best available to me were the 1962 Telefunkens. These are so rare that most people have never heard of them, or even doubt their existence. Made the Plain Jane 1961 TF 6922 sound like garbage. I bought the last NOS sleeve of early HP/TF that I saw, back in ’98. Good luck finding more.

Speaking of 6922 tubes, try some  National Matsushita 7DJ8/PCC88 made in Japan new manufacture. The have replaced my Valvo Cca's sound great and are very affordable.

Very new to this game, just purchased my first tube pre amp. Been ss forever. My question is, what is the difference in plates on a tube? My pre calls for a 12au7 long plate. Are there any other "plates"?