My sub sounds really crappy on higher volumes?

i have a alpine type r 12 DVC 4 ohms in a sealed 1.2 cuft box powered by a hifonics hfi 1000.1 with 4 gauge wiring, i am running it with 2 ohms and the amp is 750 watts rms. it sounds really good and clear on volume 1 thru 10 on my sony HU but anything past that and it sounds like shakey and not what it is susposed to? can u guys help me out, im positive that i wired evry thing correctly, thanks