Thanks for the information R.
I went ahead and did a relative tube comparison using the 11A as the reference pre and my amps. Tube changes were in the preamp.
Tubes involved were one new tube, along with some old tubes from my Tektronix O'scopes from 50s-60s, so I know there are no fakes. I measured the plate current, an indicator of Gm. All were nice specs.
The differences posted were what I would call small to moderate. Most were rather small, but the thinner top two tubes did give me the impression of near moderate difference. (Of course what does small and moderate mean.)
Soundstage was very good to excellent, just minor differences such as wider landscape.
New tubes were Genalex:
Old tubes were:
GE
Jan Sylvania
RCA (made in Japan)
Bugleboys, Holland
PQ, Holland
JJ
Genalex is a puzzle. It had nice bottom and mids, upper bass and lower mids fuller than JJs, so great flesh.
But the highs almost appeared like I had a three way speaker rather than 2 way (full range). Violins thinned out, strident, especially when installed in both the pre and amps. Just too much for me. Get that solved and a great tube.
GE was generally a little thinner across the board than the other tubes. Highs not as thin as Genalex. Not bad, but tubes below have more flesh.
PQs were a little fuller than GEs, but a little thinner than the bugleboys, Jan Sylvania, new Telefunken, JJs. Not quite as relaxed in the mids and highs. Bottom is very nicely controlled. Did sound nice though.
The RCAs and PQs were nearly identical, very difficult to differentiate them apart. A real surprise. One tube said Japan but the other I could not absolutely prove. The internals did look identical though.
Bugleboys had the most relaxed highs of any of the tubes. Almost too much. Bottom was in top form but not quite as fleshy as the Jan Sylvania, JJs.
New Telefunkens were just slightly thinner, from top to bottom than the JJs.
Jan Sylvania were a little more flesh than above tubes.
Highs relaxed while bottom nicely control. JJs bottom a little looser than Jan Syl.
JJs highs were not quite as relaxed as the Bugleboys on the highs but still very nice, bottom a little fuller, looser than Bugleboys. JJs were nicely fleshed out, very close to Genalex and Jan Sylvania.
Just my relative comparisons, using Bach Brandenburg Concertos No.3 - iii: Allegro , and a little Scheherazade.
Cheers.