I had one. This is a darkish sounding preamp so the popular NOS tubes really made the sound too thick & closed in. RCA black plates would be an example of the tube to avoid. You should try some new production tubes or Siemens, Mazda if you want NOS. You need to dampen the vibration with elastomers and adding mass. Forget about solid cones and other gimmicks.
Belles 20A Tweaks--Tubes
I cannot find very much about this Pre so I thought someone might chime in here. The pre uses 4 12AU7 tubes,I'm currently running Brimars,I might add that this unit has the Auricaps.I'm seeking information as to get the most out of this unit,as I plan to keep it.Any advice is appreciated--thanks in advance