My car amp is very soft... any ideas?

I bought a second hand amplifier... 1500w Star Sound. I hooked it up to my car 2 days ago. I hooked up a different amp a couple years back so I know I did it right. The power lead is hooked up to the battery with a fuse attached. Remote wire is hooked up to the front loader as well as the RC cables. I mounted the ground wire to a metal bracket the connects onto the back seat. I sanded down so all the paints off and its pure metal...Now I plug everything in and it works fine at low volume. I turn the gain on the amp to full and it starts distorting... I turn the volume on the frontloader to max and its still really soft and still distorts... I can describe it as this.... at low volume it sounds perfect. Clear sound, nice bass etc. once i turn it louder on either the front loader or the amp it distorts and stays at same volume. The sub woofer is fine because I used it up until 2 days ago to put it in the car. Now I don't know what the problem could be... if anyone has any suggestions please help me. Amp is still under guarantee but I don't want to take it back and they test it and it works fine. The wiring kit I used is brand new. 4Gauge power cable and all.