Someone correct me if I'm wrong here:
12ax7 are preamp tubes that have the highest gain ratings. There's also 12 at7, 12au7 and other tubes. Many guitar players like the characteristics brought by a tube that has been pushed until the signal distorts due to the tube.
There have been people using other tubes with great results as well. If you want cleaner tones with higher headrooms, go for the 12AT7 or 12AU7 (if your amp allows.) Different makes/brand of the same rated tubes can also make a difference. (WingedCs vs EH vs NOS etc.)
Again, the problem is mainly that most solidstates on the market are not built to the same quality as tube amps are. There's a Pritchard solidstate that claims to emulate tube amps, but it's at 2000++ USD. With that kind of prices, and a number of variations in building amps (different cap, resistor, wire type/brands, quality of construction, speaker, cabinet, etc etc) it's difficult to guage if tubes alone actually make significant tonal difference.
But in all honestly, don't be too obsessed by the stuff. If you play an amp, it sounds good for you, then it's good.