sraber brings up a good point.
you could be overcompensating for less than ideal sub placement.
i think, back it down,see if the sound quality clears up,should be able to play deeper also(with headroom)although not as loud
then start experimenting with different placement of the sub
then start pondering on a second sub
i just think you are overdriving,most likely there is nothing wrong

how big is your room?are you like me, a major bass head, and like to push the limits?