12AX7's all have the same gain (amplification factor) of 100, plus/minus about 5%. 'Golden Ears' types will tell you different, but the measuring equipment tells no lies. There are several other tubes that you can try without making any mods to your amp. The beautiful thing about preamp tubes is that you don't need to rebias anything. Just make sure that the pins connect to the same internal parts of the tube.

You can try 5751's which have a gain of about 70. This gives you a bit more headroom before distorting, and they are reputed to sound good when they do distort. SRV used them.

The 12AT7 has a gain of about 60. The 12AY7 has a gain of about 44.

With a gain of about 20 is the 12AU7 which is easy to find. Ampeg used them for some of their amps, usually as the driver for the power stage.

All of these tubes will simply plug into the socket the 12AX7 was in - no fuss. Just try 'em out and shuffle tubes around until you find a combination that suits your tastes.

I find this site usually has data sheets for tubes I want to check out. Compare the size and pin connections with the 12AX7 data.