This is just my opinion, and nothing more, but I have tried changing out 12AX7 pre's in my Einstein combo to 12 AT7's with great success*. (Success may be defined as only applying to one's ears if the tone is desirable.)
When I do change out 12AX7's, they are replaced with NOS Mullard 12AT7's, or RFT, Telefunken, or a similar high-end tube.
I have found that I am getting less distortion and am able to turn up the gain higher while achieving a sweeter, less compressed type of gain that suits my tastes.
Being able to turn up the gain further opens up the amp more and allows it to breathe and get tones that I otherwise would not be able to get if I had a 12AX7 in V-1, and/or V-2.