http://www.crutchfield.com/S-OojlhwAM3nf/p_575PBR3004/Rockford-Fosgate-PBR300X4.html buy one of these.....unbelievable SQ for the money
Headunits with only line level outputs are generally the higher end stuff, 1 pair of preouts will be dedicated subwoofer, with the other 2 used for front/rear.