12AX7 gain is a real misnomer. A 12AX7 should have a gain factor of 100.
Some preamp tubes end up having gain factors from 105-115. These are higher gain preamp tubes. The problem is that there is only so much usable gain in a tube and in a preamp circuit that you will only benefit from a high gain pre to a certain extent. You may get a your gain a bit earlier on the dial, but you can only drive it so much before it starts to compress. Combine this and the tendency of each preamp tube mabufacturers 12AX7 having it's own distinct tone and you will propably end up trying dozens of different combinations, including power tubes to find a tone that agrees with you. In some cases, depending on the amp design, you may never find it and have to deal with what you have. As you try, you will hear the differences in gain, brighter/darker, touch sensitivity and harmonics. the same goes for power tubes, but you can add compression and output power to the mix.
Enjoy the tweaking.