glowing red output tube, but don't think it's red-plating

I am not an electrical/tube guru, but I believe that if something failed within the tube (but not the amp) that caused that tube to draw way too much current, that would affect the bias on that tube and cause that tube to red plate.  And I also think that you did the right thing by turning the bias all the way down before powering up with the replacement tube, even though you were interested in what your bias of your amp had been doing with the bad tube in it.

I take it from your post that your amps have one bias pot for the whole shebang, as opposed to biasing each tube individually?  Again, I am not a guru, so I may be wrong on this part, but if that happened and then the tube actually died, I think that the bias reading (which would have been reading all tubes on that pot) would then have been quite a bit low. 

If you can bias each tube individually, I guess you could put that tube back in the socket, back the bias for that tube socket all the way down, and then turn the amp on and slowly advance the bias and see what happens with the reading on the meter, but if things are running good right now, I wouldn’t--I’d just accept that the tube failed and move on. 

Thank you @immatthewj ; that is very helpful.  The amps do have individual bias pots for each output tube.  I certainly agree that the best course forward is to consider the bad actor tube as not fit for service and to be willing to move forward in ignorance of why it did what it did.  I do have plenty of KT77s on hand and the effected amp is happily singing along.  I was simply interested in becoming better informed about what electrical condition could cause the described failure.  Full disclose, was hoping Ralph would chime in; he must be among the world's experts on tube systematics.  I do still have the tube and will make it's photo observable if sufficient force is exerted upon me to do so.  But I'm lazy; lotta effort would be needed.

 The amps do have individual bias pots for each output tube

Are they ARCs?  I know that some ARCs are biased by each individual tube.  Not that it applies in this case, but I used to have a pair of ARC VTM 120s and they sounded absolutely fantastic, but when I turned them on I would cross my fingers and grit my teeth.  But those amps each had one bias pot for all four 6550s.  

I don't know how much time @atmasphere spends on here these days, but he may respond if his screen name notification shows up.

No, they are two of a kind monoblocks.  Complicated history, but short version: customer asked Gary Dodd in about 2014 to build the best amps he can.  He designs these and starts building them.  He dies.  A friend and colleague of Gary’s inherits these amps and builds them out.  I buy them.  They’re awesome.  Just had to modify the individual bias circuits to stop those dramatic output tube blows.  The amps do run at very high voltage, very low capacitance.  All is good now though.  Just curious about how a tube does what the tube last night did.  I’m OK with remaining ignorant though; it’s my most familiar state 🙃 

 I’m OK with remaining ignorant though; it’s my most familiar state 

You and me both.