JAN 5755 Tube for 12AX7?

Correct about the 5751 not needing any adaption (I'm sure 5751 was just a typo).
I haven't experimented with the 5755 though I have read about other's opinions and wasn't particularly motivated to explore them. Mostly because my customers expect things that are rock solid and I can't play around with items that may or may not work consistently. For those looking for something to play with, it might be a fun gamble.
One warning about adapters; they may cause fitment issues in mics and other gear, particularly if the tube socket is collared. I've sometimes had problems with adapters and noise in extremely high gain positions, like a mic pre input.

As the "good stuff" becomes more rare I'd undoubtedly have to work at using adapters and finding good 12AX7 subs because the modern production just hasn't come along the way I was hoping it would have by this point in time.