Is this describing a blown tube, or two blow tubes?

1 What the helllll are you doing without your own basic tube tester?

Verify a tester has a socket and tests your tubes if they are uncommon.

sorted high price to low price

https://www.hifishark.com/search?q=tube+tester

accurate 151 (I have 157, gave my friend 257, gave another friend my bigger heavier, thus less portable Hickcok (it always/still does agree with my little 157).

https://www.ebay.com/itm/115853063390?mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338381866&toolid=10001&customid=d7af0598-26f6-11ee-b5f5-636437633039

2. One blown, DO NOT use the equipment until blown tube replaced. More than likely 2nd tube was ok till you blew it.

3. IF only 1 blown, get a new matched pair, keep the good one as a spare untill your new matched pair arrived.

If/when a tube eventually weakens or blows (most last thousands of hours, but I check all mine, with my little tube tester, preamp and amp, bi-annually).

4. IF, again, a tube blows too soon, there is something wrong with the amp.

5. Test matched tubes you buy, test tubes when a system problem occurs, to avoid guessing.

Last night there was a very loud POP and one of the tube went bright so I moved quickly and turned of the midmonos.

The amps ran great this evening until just now, and another POP and bright tube, but it seemed like it was the other tube, but I can’t be sure. This was followed by a low whistle or hum and I turned the amps off again.

"Bright tube"... RED PLATING?

Does the amp auto bias the power tubes or does the user have to manually adjust the power tube(s) bias?

I wouldn’t take a chance with the JJ e34 power tubes now. If they did Red Plate they more than likely are done for. It is possible circuitry may be damaged in the amp as well. You may need to have a Tech check out the amp.

Example of a Red Plating tube.

https://www.youtube.com/watch?v=6Ecrh5mglZc   

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http://quicksilveraudio.com/wp-content/uploads/2015/08/MidMonoManual.pdf

OUTPUT TUBE BIAS SETTING
Output tube bias should be set immediately when new tubes are put in; then again in 15 minutes;
and periodically the first few times of using the amplifiers. After that the bias should be checked
monthly.
The bias controls are located between the output tubes near the power and output transformers
To adjust the bias you will need a slot-head screwdriver. Adjust by turning clockwise to increase
bias; counterclockwise to decrease bias.

could also be small particles left over from manufacture that get into the plates tapping the tubes with a screwdriver handle, quite hard too, obviously not too hard to break them. may dislodge the particles. roll the tube around a bit in your hand to see if you can see hear anything moving around inside the tube. its not uncommon to find small bits left over inside the tube these can get into the plates when in shipping for example.