Microphonic tubes or amplifier glitch?

I posted before about multiple failures (or, rather, whims) of my Nightingale ADM-35 integrated amplifier, but it's been working nice during the last two weeks.

So I thought I can try the vintage Mullard CV491/12AU7 tubes because the RCA 6189’s, which did not cause any problems, sounded a bit bright. The Mullard tubes had no more than 20 hours on them. At power on, one tube flashed and there was a hoarse cracking sound from the left speaker. I switched the amp off immediately, let it cool down and inserted other tubes - a pair of Mullard CV 4004's. Everything was OK, I listened for a couple of hours, but when I was switching the amp off, there was a hoarse sound from the right speaker. Today in the morning, I listened for hald an hour and switched it off without any problems.

I have no tester to measure the tubes. Does this all mean that my small 12AU7 tubes are microphonic to an extent? Or the amp is too delicate and reacts abnormally to tube changes even if they are quite good and strong?
That flash is a mark of a true Mullard. Unfortunately the crackling is the sound a tube makes when it is dying.
Your tubes are not microphonic, but they might have a poor connection or carbon on the grids. Try cleaning the pins and very gently tap the tube on a firm surface to dislodge any deposits that probably aren't even visible to the naked eye.

The orange flash upon energizing is characteristic of some European signal tubes, and is completely safe.
Thank you!