You need to be careful with using a 5751 tube in place of a 12AX7. The 5751 has only 70% of the gain of a regular, typical 12AX7. These work GREAT in some gear, but it really depends on the circuit design. If they will work, they are indeed excellent tubes.
For new production 12AX7 tubes, I have used and really like the Mullard reissues and for a little more money the Psvane 12AX7s or the Sophia 12AX7s.
For new production 12AX7 tubes, I have used and really like the Mullard reissues and for a little more money the Psvane 12AX7s or the Sophia 12AX7s.