Out of phase pickups?
I have a Jazz-style bass with the standard two-volume/one-tone control.
When both pickups are on full, no problem. When either pickup is turned completely off, no problem. When the bridge pickup is turned down gradually, no problem. However, when I turn the neck pickup down slightly, the tone becomes thin and nasally. It sounds as if the pickups are out of phase.
Based on what I have experienced, if pickups are out of phase, the sound is thin when both pickups are on full.