OP - clipping would do that. How did you set your gains?
1/4
1/4 gain does not mean 1/4 power. Gain is essentially input sensitivity: a weak signal will a pretty high gain setting, while a strong one will need a low gains setting before the amp clips.
Get, or find a friend that has either an oscilloscope or DD-1 to check for clipping from both the head unit and the amp. Set your amp gains, then pull out your mutimeter and turn the gain knob down so it is putting out half the voltage it was doing at maximum (clean, unclipped) power. Then you know your subs are getting ACTUAL 1/4 power.
I'll see what I can do, aside from the o-scope or dd-1 since that's well out of my budget limit and I know of no one that has one, is there a way I can tell I can tell for clipping?
Are your mids and highs going through an amp or just off the deck?