I agree with jss... in my Mid Monos a 6922 that may not be really quiet in a Preamp will work fine in the driver pos.
I use JJ in there now and I think its the input tube that make the most difference in these.
I have some early 60's RCA 12ax7 at the input and these amps have never sounded better. When I bought them at about a year old the former owner also really liked RCA because there were early RCA 12ax7 in there.