Of course it is pretty hard to predict what anyone one person would prefer sonically, so at best, we can only make some recommendations on what to try.
I agree that 5751s are worth trying. They have lower gain than 12AX7s, however, so they may not be true drop-in replacements. I don't know if it is true, but, I've read of instances where the difference in gain adversely affected the phono equalization.
If I had to pick one tube as being almost always an improvement over current production tubes (i.e., a universal upgrade), I would pick an Amperex Bugle Boy 12AX7. These are expensive, but, they deliver a rich, full-bodied sound without being murky or sluggish sounding.
I have tried a pretty wide range of 12AX7s and 5751s in my phonostage. For my particular taste, and given the other components in my system, my personal preference is for the Telefunken ECC803S. But, I can see how this tube would be too lean and aggressive sounding for some systems. This tube is on the opposite end of the spectrum from something like the old Mullards.
