Hey everyone, I'm afraid the work hasn't yielded any progress. :/
I swapped the capsule for 2 strings of ceramic capacitors and the hum stayed the same. (Interesting, the caps are crazy-microphonic, but that's unrelated.)
I suppose that's good news, because it means the capsule is fine.
I also tested resistance in 15 different places around the mic PCB and body; everything is connected perfectly.
I noticed that the solder used is very strange. It has a higher melting point and always looks dull. In several key places (like where the main PCB meets the tube daughterboard), I have removed the solder with desolding braid and replaced with good ol' leaded solder.
I have, of course, tried several tubes, including a 12AT7 (lower gain than the default 12AX7) and the hum is always present. When the mic is powered up without a tube installed, output is silent. I re-capped the PSU a few years ago when I first noticed this problem. (Before I put this mic out of action.)
I've noticed that the mic is particularly susceptible to EMI when the outer shell is not installed. I discovered this while testing as I had it on top of my racks without it's outer shell installed and the hum was incredibly loud. Moving it around (rotating 90 deg, etc) reduces that *extra* hum, but the 'baseline hum' never changes.
The only 'new information' I found is that when I power up the mic, there are a few seconds of silence as the tube's heater gets up to temperature to start inducing the little electrons to make the jump, then there is a short bit of white-ish noise, then the hum fades up.
Any suggestions of what to try next? I can send a sample of the hum, including the start-up noise.