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

Inna
yea probably some coloration trades taking place like swapping boutique capacitor, resisters, etc.....
i was thinking a nice CA or WA red wine....
but the French gotta eat too

But for me, unless too much, better this kind of coloration than 'coloration by omission '
done by transistor amps.

Another vote for Vintage Tube Services !!! Andy is great to deal with and is also super knowledgeable.

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.