Recently all the bass just quit in my speakers, front and rear. The tweeters on the doors work fine and the rear tweeters work fine. There is no sound other than what is coming out of the tweeters. I checked to make sure power was getting to the back amps, and it was. Do you think that it is both back amps? Would they both go out at the same time, or is there something else I should check. I also replaced the head unit with another Mach 460 unit, and checked all the fuses and they were fine. Please help!