My experiences and opinions are pretty similar to
@sloppy_phil 's.
I tried every different 12A_7 tube I own in my Finally to find my favorite (and because the circuit of the Finally is excellent to critically listen/audition different tubes in), and I also very firmly settled on a 5751 which hit a good sweet spot to be able to use my small stable of basses, synth, and guitars all either clean, edge-of-breakup, or nicely non-linear/distorted.
I also really liked the 5751 in my Slampegg Bee, but ended up going back to 12AX7 in that one just because it's baseline output was so low, I felt it ultimately benefitted from the extra gain of the AX in my signal chain.
Personally, I'm always a little baffled by how many people like to use 12AT7s and 12AV7s to reduce gain in a 12AX7 slot... very consistently in my experience:
5751 is a much better sounding/responding medium-gain substitute than a 12AT7, and
12AY7 is a much better sounding/responding low-gain substitute than a 12AV7, and
I feel that really makes sense too when you consider that the AT and AV were originally designed/intended to amplify radio frequencies, while the AY and 5751 were intended for amplification of signals in the (human hearing) audio frequencies... the AT and AV don't do a
bad job by any means, they just don't seem to please my ear as much in critical listening tests (though of course once the output is mixed with different instruments, that minor character difference
does mostly evaporate)
Jack