It's difficult to tell remotely, but it does sounds like you ran into a bad tube. But since the tube worked for some time, it's somewhat possible that somehow your replacement tube didn't make good contact in the socket or some other odd thing went wrong.

As for output tube re-bias, you don't need to do it unless you change the output tubes so you are OK there.

You should be able to get a replacement 12AX7 at the store where you got it from. Typical guitar stores wouldn't have the equipment to test tubes (a number of them orders from people like me), and big franchises, as you know, couldn't care less. But if it's a friendly shop, you might be able to get them to try the replacement tube in their house amp for a while. At least that'll give you some level of comfort.

If this problem does happen again with another replacement, I'd take the amp into a tech though.

Aki