Jolida jd-502p

They work fine. Will you like them better? Give them a try.

I would simply try them out. In virtually every instance that I rolled a 5751 in for a 12AX7 it worked. The different sonics may or may not appeal to you but I really like them. You may want to start with recent production NOS JAN 5751s from the 80s they shouldn't cost very much. It will give you an idea of what the better ones sound like. Have fun trying them out.

The 5751 is simply a lower gain version of the 12AX7. It will not do any harm in the circuit and is a direct replacement.

Like it better? Only you will be able to make that decission.