vintage '50s 12AX7: Mullard, Amperex BB, Brimar

You might find this helpful. I have experimented with a number of 12ax7's and have settled into the Brimar CV4004's. As you'll read through many of these threads it really ends up being your ear that drives your final decision. Best of luck.

http://www.audiotubes.com/12ax7.htm

I really like the Mullard CV4004's. Indeed, all of this is personal preference and what tube works best in that piece of gear's circuit design.

http://www.upscaleaudio.com/Mullard-CV4004_p_5.html