Tube info

Here is another link with some great info on the different 12au7 tube types and manufacturers, and on how each brand sounds.
http://www.audiotubes.com/12au7.htm

Japanese NOS like NEC are very good.....basically no noise that i can discern, and cheap.  Of course NOS RCA 5814A as well.

Jump on Amperex NOS asap they are the best by far.

But none of the European brands are cheap...

In a power amp the tube or tubes in the gain stage have the most influence on sonic signature. I've been told that the driver/phase inverters have little effect on sonics; eg, timbre, dynamics.
Not so, IME. The tubes in the driver stage have an audible influence on the sound (depending on the design of the amp). 

I'm in agreement with all the comments thus far to choose a good NOS tube for this stage even if it's purpose is simply to drive the power tubes.