Yeah, I guess it depends on whose kit you're using to build. In the case of the micparts kit, it's not a trimmer but a specific resistor value.
That's their fault they are not copying one of the copies and using the original design that you have to match everything for a mirrored pair. Other people that use that circuit use a trim potentiometer so they can match them.
R3 is for biasing, lowest possible THD and symmetrical clipping. It will somewhat affect gain, but not to the extent described in the OP. Judging by the voltage measurements, there was nothing wrong with that resistor.Looking at the schematics, R3 should be adjusted to match gain.
Also, if you use a single turn potentiometer, make sure it goes to the other pin. Some will have resistance at the bottom of the wiper differently from each other. This should be a multi-turn potentiometer anyways.
Hard to say. If I was into building them I would get a dozen or so of those fets to match them for a set. That is why I think some kits are not that great sounding compared to someone getting one from a boutique builder. Because I think everyone including me would use different parts than what microphoneparts.com does .
According to MP, the JFETs for these two microphones were matched and biased before they were sent to me (part of the extra charge to get a matched set). So ... probably just something I did wrong during installation. Strangely, the mic with lower output was the 2nd one I built and I was even more careful not to use too long of heat when soldering in the JFET. So I dunno what could have gone wrong. Maybe there's some other component that's not good. We should hopefully find out within a week.
JFETs can change when you solder them into the board. Because I've been there with JFETs before matching it for a 1178 build and tried using a test circuit to match them.
You can substutute the xformer for NTE10:3 and save a tad as well.
