FET input problems aren't unusual on most Dumble clones. The FET was originally incorporated as a preamp for super-low output piezo acoustic pickups and not intended for use with traditional magnetic pickups. My guess is your magnetic PU's are overpowering it and causing it to clip. There is a trim pot to dial it in, that may have been mess with as well.
You'll find quite a few threads on here about the FET input. Including one where I added this...
The FET input is generally ignored and considered obsolete, you won't even find a FET input on Fuches, Two Rock and most other high $$$ clones, and if you even get a chance to look at the real Dumbles being used by Robben Ford and others, you'll see the input taped over to avoid being accidentally used. It was originally intended as an in-line preamp for very low-output PU's like acoustic piezo's....
I'll add that even Nik scraped the FET input on his HRM models.
Maybe one in ten users ever use it.
to keep the forums alive.
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