I agree with Rcj1231 who said "Sylvania 5751, triple mica black plate or Sylvania Gold Brand 3 mica grey plates. They made a big difference in my Jolida, nice and warm!" I've used both in my Jolida integrated amp and in my California Audio Labs Alpha DAC. I give the nod, ever so slightly, to the black plates version--vocals, brass and vibes pop at you and melt around your ears. I'm using Sylvania black plates right now, in DAC and preamp, and Miles Davis is sounding pretty good in the background. The GE 5751 black plates are excellent as well (much, much better than the grey plate GE JAN 5751s) and I like them somewhere in between the 2 Sylvania 5751 versions. You might want to try Raytheon 5751 black plates, although there's seemingly multiple variants; the best are very tight. I can't get behind RCA 5752s--they don't sing. Mullard 12ax7s are too fuzzy sounding for my tastes as are Telefunkens (more fuzzy than the Mullards I've tried). Bugle Boy 12ax7s are pretty good though if you want to stick with 12ax7s instead of 5751s. Have fun!