I'll let you in on my take on these crazy little things called tube sound:
As good sounding toobs supply dwindle (e.g. Siemens, Mullard, Holland Philips, RCA, Telefunkens, Valvo, etc.), they get replaced by latecomers from china, ex-soviet, etc. or some bottomend rejects of telefunkens, etc. where they are inherently noisy or not up to spec.
Because of that: anything requires high-gain these days, I resort to using either FET input op-amp, or some high performance op-amp to replace previously designs such as RIAA tube gain stages, etc. to eliminate the discrepancies between gain stages, or gain stages that are sub-spec....noisy, structurally poor (sustained ringing is fairly common), etc.
Only on line stages, etc. where medium Mu tube are widely available and good ones are not that difficult to attain. While the common ones such as 12AU7, 6SN7, etc. are at the same fate as high-mu ones like 12AX7, there are much more good stuff in this category that can still be had at a very reasonable price. My initial fave was 5965, but then I discovered that 6N1pi was the "2-die-4" for me and I have been forever hooked on it a decade ago.
power tubes, well, too bad the good ones are long gone, so now you have to live with some sino made crrraabp or some ex-soviet stuff that sounds hard and cold. Good quality EL34s from Philips, Siemens, Mullards, etc. are priced astronomically, and 6550 Generax Gold Lions are tough to come by...
Oh well: I still have some RCA 2A3 bi-plates that I live with, and should last me till I kick the bucket...
Q.