Subwoofer Amplifier Problem

I have run into a problem that I am having trouble diagnosing. I have an Alpine 9813 head unit, an MTX 895 5 channel amp, diamond audio components in front and 2 infinity kappa perfect 10s in the rear of a single cab silverado. It's always played great, never had any trouble. About two weeks ago the subs stopped playing. I checked and there are no shorts and there is continuity on all wires. I tried to measure voltage on the subwoofer rca preouts from the Alpine with a DMM, but I'm not sure how to do this. I'm struggling to figure out if it's a problem with the amplifier or the preamp output from the head unit. Oh yea, I can turn the gains all the way up on the amp and the speakers will play, but it is not right. All of the musical sound is not present. I have a 24db/oct crossover set for the subs at 80hz. The diamonds still play perfect. If anyone could point me in the right direction, it would be much appreciated. Thanks Shaun

1. Make sure your sub out is on on the deck an turned up. 2. Switch your RCA's around. don't worry abuout the x-overs for this. This will determine if your deck or rca's are the problem 3. swith speaker leads, again don't worry about the x-overs right now, just see if it is the amp. 4. remove the box and try it with a known functioning amp. 5. check impedence on the subs ( usually done earlier if possible)