Good to hear that you resolved your problems. My bet is that the tubes were going bad. But, it could also be the case that the 5751s are superior to the OEM tubes. Manufacturers must go with readily available tubes, in order to supply replacements to customers, even if there are superior alternatives. Readily available usually means in-production tubes.
With my Audionote amps, good vintage tubes were supplied by the manufacturer, but, these were somewhat common tubes (RCA 2a3s and RCA 5692s) and nowhere near the best possible tubes for these amps.
My phono stage uses 12AX7s too. I preferred the 5751s I tried with this phonostage over the manufacturer-supplied tubes, but I like Telefunken ECC83s and ECC803s even more. The ECC803 is, however, an extremely rare and expensive tube.