I agree with trying 5751s. I prefer the 5751 in most cases where the somewhat lower Mu is compatible with the other elements most people run. . In your case I think it will work just fine. I think your jaw may drop further with them, because I find them to have an even larger and more imaging - soundstage.
The caveat is that 5751s produce about 70% Mu compared to a typical 12AX7. That means that they may not be sufficient to work with other parts of your system. High gain phono stages where the gain is critical, such as in an MC stage.