distortion

For some reason when I turn up my punch bass it distorts tremendously. I have a rockford fosgate 250.2 and a 500bd. I am running a stock cd player with momo components off the 250 and a momo 12 off the 500 bd. I just got everything recently and it was all brand new. Maybe not the most professional people installed it but they did it right, i think. They said it was the sound of pushing alot of air, but it sounds like crap to me. i turned the sub around, so it faces toward the seat, and it helped alot. they box is a .85 qlogic type 1 and its all in a 2001 frontier crew cab. I listen to rap and didn't realise i picked the wrong sub for my music, but i don't want to have to change it because I think it has alot of potential. Would a bandpass box be alot better? Any help would be greatly appretiated.
Sincerley,
a desperate and frustrated polk fan