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
The best tubes that I tried when I had my 20A were the Special CJ Test version of the Mullard CV4003 from Upscale Audio, (recommended by Kevin Deal from Upscale). They were "magical". Don't know why, but they did sound better than anything I tried.

I also used BDR cones underneath and either a VPI brick on top or a Brightstar Audio Little Rock on top.

BTW, the 20A was one of my favorite preamps. It's very musical and smooth and open...I really miss her...
Thank you kindly for your reply. I currently have some 1950's Raytheon Black Plates,,I'm gonna take your advice and try the BDR cones and Mullards.I appreciate the time you took to respond--thanks---George
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.
Thanks for the reply and advice.Based upon your sugestions,I jusr rolled some JJ goldpin Ecc802's in and so far I am very pleased with the outcome. The pre seems more musical with a tremedous soundstage.Lively, but not bright,it is as if she "woke up".Souns very detailed but still just a tad laid back,which is exactly what I was looking for.Thanks for taking the time to respond.