I have a Mike Brown bass with Nordy Big Singles and an Aquilar OBP-3. Works great until I turn the bass past half to 3/4. I get this nasty high pitched distortion when I pluck a string. Anybody?, the bass is super-deep and powerful anyway, so I don't see myself turning it up past the halfway point, but would like to get it fixed if there is something wrong.
When I read OBP-3 Distortion, I was thinking of the BOSS effect pedal It's possible that it's a grounding issue, or you should apply copperfoil on the inside of your pickup covers and the inside where your preamp is, that can fix it for you
Are your batteries in the preamp shot? Are you just overdriving the input stage? Can you use a buffered input or turn the gain on the amp down?