My b keeps cutting off at less than mid volume.?

Ok so i have 2 kicker L5's dual 4ohm and running a PPI DCX 1000.1 amp, when played at a very low volume the b stays on but when i turn it up not even half way the b cuts off, the amplifier starts to clip out, i have no solution why this is happening and is getting me very fustrated, i also have a 1.2 farad capacitor. running 4 gauge wire.