Indeed, it would be best to check with Quicksilver first. But keep in mind that some designers will object to you tampering with their perfection, despite the fact the swap will not damage the circuit. The two types are too similar for dire consequences to result (tubes have very loose tolerances anyway). The amp will simply not sound exactly as the designer intended it but, as the owner, which sound is better is for your ears and system to decide IMO. Just like rolling tube brands or sending your amp off to have mods done to it by a third party.
However, I do want to state for the record that using a 12AX7 in a circuit designed for 5751 is NOT a good idea. But the reverse is technically ok, if perhaps not subjectively so.
Arthur
