12au7 tube?

I've heard good things about the Apos Ray tubes. They're not cheap, but they're less expensive than a lot of the NOS options.

You may try CryoTone tubes. In my experience with them, they are simply quite. Also, they offer a trial period. If you don't like them, send them back. 

I love the Brimar CV4003 from Upscale. $300. I own Tele red medicals from Brent, Siemens nickel plates & currently prefer the Brimars. To be honest, I haven’t tried my Tele’s yet in my new BHK pre. They were my least favorite in my old Rogue. YMMV.

Well why not try the Telefunken 12AU7? CBS 5814 are good as are the Amperex. Really depends upon which direction you want to take the sound. I always found the RCA clear tops primitive and bright sounding. 

I’ve run 12AX7 and 12AT7 as signal tube in my integrated.  No experience with lower gain 12AU7, however I have found consistent sound signature across mfg with AX7 and AT7.  Mullard = warm, smooth with less detail, Telefunken = master of transparent balanced detail, Amperex BB = clear, clean sparkling treble extension. My favorite that combines all three are Raytheon black plates from 50’s = warm, strong bass and midrange with great treble and detail up and down frequency range. Doesn’t have full extension and sparkle of BB, but has warmth of Mullard, but not too warm, detail and transparency of Telefunken but not at expense of bass, and superb treble extension but doesn’t leave out beautiful midrange. Goldilocks of signal tube in my system.