I still consider Joes Tube Lore the best resource for information on tubes: AudioAsylum.com -- Joe's Tube Lore. And since we are talking NOS tubes its description is always current.
In my preamp, of the many brands of NOS 12ax7s and 5751s I have used, I prefer Sylvania gold brand black plate triple mica 5751s.
I agree with Joe; in that to my ears in my preamp 5751s sound better than 12ax7s. Very subjective

