12ax7 tubes can be enterchanged with xxx tubes?

I had been using Jolida CD Player (please see my system page)with RCA Blackplates 5751 and Sylvania Gold Brand 5751 for almost 8 years now and counting. You will only "hear" the change in gain if you are using the CD player direct to your amp without a volume control, otherwise, if you are using a preamp, it does not matter and a few clicks/small turns to open the attenuator of your main preamp is all you need.

The 12AU7's or the 12AT7' similarity to the 12AX7 is the pin out and the glass envelope, it should stop there.

Try 5751(RCA, GE, Sylvania). Once you hear it on your CD player, you will never use a 12AX7 tube in this particular player again. Oh, I measured the output of the CD player using 1kHz test tone before to compare the output to that using 12AX7's, I believe if I recall correctly, I got approximately 0.9 to 1.2 volts for 5751 depending on the input impedance of its load. The 12AX7's was about 2.15V I think.

It's my understanding that the 12bz7 might be compatible with the 12ax7 as well. You certainly want to check and see if they will work in your piece though.

The tube most likely to give you the warmer tone is an NOS Mullard. The Blackburn Mullards and all of the 50s to 60s longplates will be your best shot. I should forewarn you the market is rife with spent used pulls that have questionable performance. The Jolida does not put a lot of demand on the tubes so you may get away with a couple of weak ones.
In the out put section of the Jolida CD player I was quite pleased with the 80s Mil Spec Mullard which used to be available here on the Gon by the same distributor of the KR tubes. I think it was the CV 4004.
For a wide sound stage I agree that the 5751 is the ticket. I would stay with grey plates as they always seemed a bit warmer to me. For Bass emphasis in particular I would opt for the GE, with the triple mica being preferred. The 5 star, the JG mil spec, and civilian all possess abundant bass. The 5751 Blackplates of all brands are in general a better all around tube IMO. They have taughter bass and cleaner treble.
As you have been told it is always a matter of taste. If you want to save a bundle, and just get a sense of how a 5751 might sound, I would certainly try the current production Sovtek, which the dealer will back to boot. It doesn't hold a candle to my and most everyone else's favorite Triple Mica Blackplate 50s vintage Sylvania, but they are virtually extinct as true NOS. The cost for good pair is prohibitive and may not be appreciated in that application.
FWIW only a 12AX7 or it's known close subs like the 7025 6681 and 5751 functioned in my CD player. I.e. the 12AU7s nor the 12AT7s simply did not work for some reason.
Good luck

5751 should work fine. 12AT7 might work. I wouldn't try 12AU7.

I like the 1950s GE and RCA 3 mica black plate 5751s.