Just purchased a Godin guitar, Piezo pickup, not a humbucker - whoops, get incredible feedback when I plug it into my Spider 4 120. Turning volume 'way' down (too low), moving further from amp helps. Any suggestions please?
Sounds like it may be a grounding problem on the soldering job. I'd do an ohm check for resistance across the pots and jacks and switches. Being close to the amp, or having the volume up will exagerate the noise, but the noise is probably coming from a short somewhere in the loop from the pick up to the jack.