Sorry for my lack of knowledge on this, what exactly does this mean? And does this change how I should approach swapping out the tube? Thanks!
Simple answer, no, it's ok, swapping out the tube is just that, swapping out the tube
(I like to make sure I don't leave any finger prints (oil) on the tube, drummed into me by lighting guys when changing PAR64 bulbs so a bit pedantic no doubt)
Yes it uses a 12ax7 but it's used as a push-pull amp with it's own output transformer.
So any 12AX7 will work (noting they all have their pros and cons), it probably is there to add tube happiness (a bit simplistic, so here's my try at an explanation (again MJMP pls correct me if I'm wrong)), running in push pull means each of the 2 triodes (within a 12AX7 tube are 2 triodes (gain thingys) which can be wired in various ways) which, in push pull, are each amplifying one half of the signal +ve / -ve. This is probably part of the speaker simulation as it can imitate speaker cone break up without some of the harshness (raggedness) that comes with the real (mechanical) thing (breaking up) (e.g. reduce 2nd order harmonic distortion et al (much broader topic...)). I suspect the output transformer is then used to make the signal mixer/line input friendly (impedance etc).
It actually works like a push pull power amp but with a preamp triode.
So, (to expand on MJMP) like the output tubes in most guitar amplifiers, 1 or 2 tubes amplify half the signal +ve, the other 1 or 2 tubes the other -ve. The 12AX7 is kind of like having 2 (power) tubes in 1 but at line level not speaker level if that makes sense. So you can run it in push pull (like many power amps) and gain any perceived acoustic benefits.
Cheers Richard