Stan,
Actually, if you test it with batteries both pickups will work if you plug stereo headphones into the 1/4 jack, no mod required. Neck pickup will be in one ear and bridge pickup in the other. Also, if the bass has LEDs on the neck, they probably won't work without the power supply. The power supply feeds +9vdc and -9vdc to the bass, and audio returns unbalanced stereo through the cable. I'm sure there is a schematic on the site somewhere that has the pin-out information, and either Keavin or Dave will probably post a link to it. The Alembic power supply has the audio breakouts (Neck, Bridge, Both) and is an elegant unit.
John