Personally, I like to get away from having all of a preamp being 12AX series tubes. Granted this is not anywhere near as easy as subbing in another AXes, but to me the AX7 in particular is not a standout good sounding tube. Even a 6AV6, supposed to be the functional equivalent of half an AX7, sounds a LOT better. My heart truely belongs not to dual triodes, but the combo pentode/triode tubes like the 6AN8 and 6WJ8, but those won't work in a circuit designed for an AX.
I did come across something interesting, that I think I will explore further in the future, which is yellow-jacket style adapters for preamp tubes. I recently found available from shops online that I order other things from anyway, both tube bases (that I could wire up myself as adapters) as well as some premade adapters (e.g. 12AX7 to 6EU7, which I have never used/heard, but now am curious about). They are readily available (NOS and new production). Supposedly really low noise, but I dunno if they sound any good (not common, but apparently they have been used in guitar amps).
I did come across something interesting, that I think I will explore further in the future, which is yellow-jacket style adapters for preamp tubes. I recently found available from shops online that I order other things from anyway, both tube bases (that I could wire up myself as adapters) as well as some premade adapters (e.g. 12AX7 to 6EU7, which I have never used/heard, but now am curious about). They are readily available (NOS and new production). Supposedly really low noise, but I dunno if they sound any good (not common, but apparently they have been used in guitar amps).