Hey, folks, I have a bass with a single Nordstrand Zen Blade and the OPB pickup. It is in a well-shielded cavity. Ever since I got the bass, I've noticed a sort of high-pitched hum (not like the typical grounding hum). It doesn't matter if my hands are touching the strings or not. The only time the hum goes away is if is sitting face down, either on the ground or on my lap. When playing, it varies in intensity, depending on which way I'm facing. Few things: I've checked for bad solders, touching wires, all that, numerous times. Everything looks good. I didn't wire this bass, but I do understand wiring somewhat. It looks well-done. Nothing loose, everything's neat. It does vary in intensity, depending on which way I'm facing, but only disappears if face down. It does this whether using an amp or a headphone amp. I've tried it with all of the lights off, I've tried it outside, so it doesn't seem to be dependent on some other interference. When in passive, it's still there. Not as intense, but it's still there. I'm at my wit's end, here. I've been back and forth with the luthier proposing different things to try, and I still can't figure this out. Could it be a bad pickup or preamp?