matched quads, one dead, then what?

If you still have the boxes the tubes came in, you may find the transconductance and/or current draw number marked on the box. This is generally how tubes are matched.

If the info is there, you can call your tube supplier and ask if they can sell you another tube with the same values.

Or, if you have a tube tester (or know someone who does), the three tubes can be tested for a current value and then ask for a matching value tube.

The relatively low number of hours on the remaining good tubes should make this a good option.

Perfect - Thank you Mlsstl.

...and this kind of seemingly random, premature tube failure is to be expected once in a while? Or is it a symptom of something else in my amp. Bias is correct, and audible performance was fine until the tube went.

Mlsstl is correct assuming you actually needed a matched quad to begin with. If you have to manually bias each tube in your amp, you really don't need a matched set. The biasing process will take care of variations. Matched don't hurt though.

...and this kind of seemingly random, premature tube failure is to be expected once in a while?

Yes

One of the reasons I purchase my output tubes(matched octets) from Upscale Audio, is the fact that they record all the measurements on the boxes. That would make it a piece of cake for Kevin to perfectly match any tube that had prematurely failed. None have, in the 10+ years that I've been dealing with him, though.