A couple of obvious things to check:

Phase switch - did you knock it when you moved it? I do not hear much difference with my HSU when I move this switch, even though it's purpose makes sense - worth a try.
Crossover in/out switch - again did this get moved from out (receiver handles crossover) to in (knob on back of sub is active). This could make a huge difference depending on where the knob is set.

My Home Theater