One answer: call Kevin Deal at Upscale Audio.
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I talked to a Russian-sounding gentleman from B.A.T. (proprietor). By the way, he spoke favorably of Kevin Deal.
B.A.T. reportedly uses only current production tubes (I assume to ensure continuous and consistent supply). Both factory tubes are labeled "made in Russia."
Kevin Deal recommended some "special" Sovtek 6922s (many different versions are similarly labeled), and Mazda or Visseaux 6V6-GTs.
Waiting for delivery now.
Received 4 70's era Sovtek 6922s and 2 1952 Mazda (French) 6V6s, as described.
I like the new sound better but can't exactly finger the difference - maybe a tiny bit "harder", but more air, less congestion, improved stage. These changes are minor, compared to the next 2.
Before the change when I tapped a finger on the preamp with the volume turned up to about 40% there was a very audible "thud" and decay coming through the speakers. This was not a very hard tap. With these new tubes it is silent. I'm thinking there was some feedback (air born vibration?) into the tubes causing congestion (microphony?) at higher volumes, even though the unit is on a Townsend Sink. Anyway, it is better now. These tubes appear to be much less sensitive to vibration.
Oh, and less noise, of the tape hiss variety.
I'm very pleased with the recommendation, and would do it again.