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. 

 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.

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

You and me both.

Pretty much the definition of red plating!

Ghat's what I was thinking; I enjoy finding out that I am not always wrong.

If the grid of the tube opened up (which can happen due to poor solder joints in the base of the tube, which is a lot more common these days with Russian and Chinese power tubes) the tube can go into runaway. 

But I didn't know what it is exactly that would cause a tube to red plate; I am not the OP, but as always, thanks for the education, @atmasphere .

Usually in an amp that was functioning the main reason is a loss of bias, which is to say a loss of control Voltage on the control grid of the tube. A typical large pentode for example might have a bias Voltage on the grid that is -40 Volts or so with respect to the cathode of that tube. If the grid resistor opened up, or the contact of the tube or the tube socket failed, the control Voltage would be absent and so the tube would conduct as hard as it could- hence red-plating. IOW this is a loss of bias. 

Thank you for the continuing education you are providing, @atmasphere .

The lucky bit is tubes are pretty forgiving about this. If you detect a tube is red plating and shut things down quickly enough, the tube may well survive undamaged, assuming it wasn't the problem in the first place. 

I think I got from the OP that his issue was fixed by replacing the tube, and the system sounds good again.  Meaning that it was a tube failure, which I would think would be preferable to issue with bias due to something happening with the gear. Earlier I referred to a pair of ARCs I used to own that sounded great, but were unreliable.  Often enough that it was not a blue-moon even, on start up one or the other would blow a grid resistor and that tube (it used 6550s) would take on a strange glow.  To be on the safe side, I always swapped out the pair (or sometimes the whole quad) of tubes.  I am assuming that these failures were due to the component and not the tube, although the component malfeasance was bringing down a tube when it occurred?

 If I had any pride I'd be embarresed by my silly question, but happily I don't, so I'm not. 

“Pride goes before destruction, a haughty spirit before the fall.”

Not a silly question.