High gain or low gain 12AX7 for cathode follower?

As far as cathode follower specific tubes go, doesn't matter if it's high gain or low gain. Whatever sounds good to you. Balanced doesn't matter at all. Typically modern tubes with spiral filaments don't last long in a CF position for some reason. From a technical standpoint, what matters is the heater-to-cathode voltage differential. Needs to be at or lower than 180v to not cook tubes. That's why some builders reference the heater center tap to an elevated DC voltage instead of ground.
 
Here's a good read on DC Coupled CF we use in guitar amps
http://www.valvewizard.co.uk/dccf.html

From my experience Higher gain tubes in this stage add compression, and possibly more fizz. Lower gain tubes are more open, and looser.

Also Chinese 12ax7, JJ ecc83, and Sovtek 12ax7wa/wb/wc are reliable in this stage. Sovtek lps, EH 12ax7, tung-sol 12ax7, (probably several others) can fail here if you do not have elevated heaters.
 
I typically stick with a JJ, or Ruby Chinese in a CF position. They seem to be the sturdiest, with JJ being the best bet.

I stay away from anything New Sensor, (Tungsol, Mullard, EH), in those spots.
 
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