"Hum" is not distortion and it would not be related to the voltage. You were probably just in an EMF-noisy environment, with that single-coil bass, and the hum that the bass picked up was amplified by the compressor. This would happen with even the highest-headroom compressor.
While you examine everything to diagnose the problem, try to move the rig to different rooms, or different parts of the room, or even a different building, to see how the sound changes. Try to write down exactly how the noises sound, and whether they sound like anything else you've heard (e.g. more like electrical humming, more like overdrive, more like glitchy scratching, etc.). Does the hum/noise change when you play, like does it get worse when you play harder, does it go away when you play quietly or stop? Plug your bass in, but turn its volume knob all the way down; what happens to the noise?