preouts can clip.
now set ALL your EQ\bass\whatever to DEFUALT (flat\0), then try it again.
now, before - when he was talking about clipping - at around 22 on MY alpine, the INTERNAL AMPLIFIER will clip and distort my SPEAKERS - however, at that SAME volume, my sub never sounded distorted. so no - your subwoofer preout should not be clipping. but i had my bass EQ at -1 and my subwoofer level at like 4.
turn the freaking settings down. it sounds like your amp is set pretty conservativly, maybe wanna set that with a little more output?