12ax7 tubes can be enterchanged with xxx tubes?

the 5751 may or may not sound better to you. You will need to try them to see. 12au7 and 12at7 can be put in the 12ax7 but the circuit is not optimized for them. It shouldn't hurt anything thing if that is what your asking. Tubes are very subjective and it's hard to say what you will like. I would personally stick with 12ax7's or 5751 in that position though. There are a lot of 12ax7's to choose from or 5751's new issues and NOS.

Being that the 12ax7 is used for the gain on your CD player the 12au7 will have considerably less gain than the 12ax7.

Try the 5751 first you may like it. A little less gain but you may like them. If your wanting more clarity and nicer sound think of changing the coupling caps. Other mods can be done as well. Good luck!

u can try create audio 12ax7. it's smooth and grain free.

You may want to try the RCA 5751 3 mica Black plate...Sweet with good bass...Its all a crapshoot,but the RCA's are a nice sweet 5751 tube.........

A 12au7 has 80% less gain then a 12ax7,a 5751 has 30% less gain than a 12ax7. I would not do that change! Just try some different 12ax7 tubes,I like older sylvania or Ge you can get them cheap.