Thank you VERY MUCH for a proper troubleshoot thread. It is almost like pulling teeth at times. 😉
So the first thing I notice is you say you have the Boost bypassed but your output wiring is coming from the Output T pad so you are using the Trimmer to control the Boost Volume which is then connected to the final Output T pad. My point is the Trimmer Boost Level needs to be set at 1:00 for unity gain. Otherwise it attenuates as you describe. From 1:00 position to full up is a boost of about 15-20 dB. Looking at the notch on the T2 Trimmer it is pointing toward about 10:00 which would attenuate. I do see that T1 is full up.
Make sure T2 is set at 1:00 or higher.
Having the option of Attenuation or Boost allows the user (guitar or bass) (active or passive pickups) to get a desired setting for nearly any rig or recording situation.
If that is not the issue in regards to the Trimmer setting my next guess is the MPF102
There are too many bad MPF102s saturating the market from the last 4 years. Can you show us a closeup of the Font for the MPF102? Many MPF102 “named” transistors have been saturating eBay etc.. for quite a few years now and are either inferior or fake. I can recognize the correct Font with a closeup of the transistor. I have seen 5 different fonts (4) of which are no good.
That is all I have at a glance for now and I hope it helps.