I have found in my experience though, that tubes can be microphonic or noisy with the APV, and silent through other gear.
I am not sure why that is...
I believed for a while, heck maybe still believe, that the tube socket was to blame. The socket isn't the tighest i have seen, thats for sure.
Because, the tubes i have had problems with all fit more loosely and/or had chipped or corroded pins.
Anyway, finding a silent tube is not too hard.
Still enjoying the APV very much.