Re: 12aX7's vs. 12AT7's.
Like The only differences are gain, the headroom is an issue of the amp.
12AX7, 12AT7, 12AU7,12AY7, and another I forget are all interchangeable, the only difference is gain, AX is the hottest, I think I have the gain order correct. I'd have to look that up, but any tube amp site should have it listed.
The difference tonally has to do with how the amp interacts with it, as the compression points are a bit different, but I couldn't tell you more specifically than that though. Also the 12 series the heaters can run off 12.6v or 6.3v, but most amps run them at 6.3v so they last longer. There are also some other odd balls that can be plugged in there as well, but you'd have to look that up on a tube site.