I am curious where you get 760watts, with 380x1 at 4ohms? What if you ran it at 2ohms?
Isn't the testing inaccurate if you don't have load on one of the channels? I could be wrong though. Now since your sub is a 500w RMS, 1000w Peak, and you are only sending it 380w, there would be a BIG difference between that and 500w. Especially since your amp doesn't peak [does it?]. Personally, I would save up for a 500w amp, that has a peak of around 600-700w. Amps that peak are more expensive, but you will be much more satisfied with the results.
Also, have you confirmed the voltage AT the amp? Maybe you aren't getting a full 14.4v? What gauge wiring are you using?
'89 SSE - GX3 -190k, 155k on body - Back for another ride.