help with adding sub to mach 460
 

ok so i got all my stuff in and installed everything, have an alpine M500 amp with a single 10 inch infinity perfect dual 4 sub, the problem is that it sounds like junk the sub is just not pushing at all like it should, i hooked the sub up in my friends car to his amp and it sounded great so i believe the problem lies with the line output converter, i tapped it into the front left and front right speaker wires at the factory head unit, i would have went directly to the back speakers and tapped in but i have a convertible and it is i pain to get to the speakers, what should i do???