Banging on the trimmers vibrates everything. The may well be an intermittent short or open somewhere. Try banging on just the board, not on a component.
I think you should audio probe each lead on Q1 & R4 again. We must have missed something. First set Q1's drain to approx 4.5V.
According to the schematic, turning the trimmers clockwise makes the resistance decrease. All the way to zero Ohms (more or less) at full CW. Here's the thing: depending on which trimmer you buy, the rotation could be backwards. Bourns trimmers come both ways. I have experienced this. Either end of the trimmer rotation will kill the gain of that stage because at zero Ohms, the JFET is driving a short circuit and at the maximum resistance, the JFET saturates (or gets pretty close to it). The trimmers interact in the sense that each one affects the level & distortion of the signal driving the next stage. You should be able to dial the drain voltages to 4.5V and get some sound to pass through the board. TRIM2 might be the only exception, 4.5V on Q2's drain might be too low because R8 is relatively large. On Q2's drain, I'd shoot for 6V for starters. PF5102 should work fine in this circuit, their specs are very similar to J201. Last I heard, we're not getting any signal at the GAIN pot. If that's still the case, then messing with anything downstream is futile at this stage in the game.
