pre amp tubes

They are physically interchangable, being 9 pin minuture dual triodes. They are not electrically interchangable however, so you should probably refrain from substituting one for the other.

If you search the forum you will find the question being asked over and over again, and hundreds of great suggestions as to which is the best 12AX7 and 12AU7 type tubes.

IMHO, it is also very context dependent so you should search for suggestions for the tube types that work best in the specific gear you are using. Without knowing this the question is a bit like asking what the best tires are without knowing if you drive a sports car, light truck or tractor.

I would contact Kara Chaffee at deHavilland Hi Fi. She has forgotten more about tubes then most people know. 

dehavillandhifi.com  

or: 
kara@tubewizard.com

Thanks for that. I am using a modified (mainly Blackgate caps) YS Audio pre into a Rogue Stereo 90 power amp on the electrostatic panels of my bi amped Martin Logan Vantage speakers and an Acurus A250 on the base units. The pre came with Phillips Mini Watt 12ax7s which I replaced with Tungsram and then matched RCA NOS 12ax7s which made a hell of a difference. I would replace with RCAs again if I could find someone selling them at an acceptable price

I would strongly suggest not subbing those tubes ....   if the circuit calls for a 12ax7 use one, or an accepted equivalent.   Ecc83, ecc803, cv4004...   a 5751 or 7025 are both lower gain ax7 subs.  

I have a pair of early 60's RCA 12ax7 in the input stage of my amps and they are one of the most well rounded , balanced sounding tubes I've ever owned.   

I bought the amps with those tubes and they probably have some serious hours on them.  That said , I think I am going to try the Gold Lion 12ax7 as I have heard they are the best current production 12ax7.

Generally, you can use a 12AU7 or 12AT7 in place of a 12AX7 without fear of harming your preamp, as they are electrical similar but the U7 and T7 have lower gain. That doesn't mean it will sound the same; it might be better, it might be worse. Checking with YS Audio first is always the safest bet.

I have a Jolida JD9 that will use all three, and with MM cartridges I prefer the 12AT7 over the stock 12AX7. The 12AU7 just doesn't sound right regardless of cartridge.