amplifier channel testing

I need to check the channels from my amp, and I am not really sure on how to do it. I know I need a DMM, but I am unsure on what setting to use on it, any help?  

The reason why I need to test it is because, a while ago I had a problem with the subs going on and off, I pulled them out of the box, checked all of the wiring and they where ok. Put the box back togheter, hooked them up, and they worked fine.  Now they are doing the same thing.   Last night I turned my stereo on with the song I left playing in the afternoon, and the subs where barely there, so I turned the bass remote knob (rockford) up and they came on really loud, turned it back down and the subs where working like they did in the afternoon. So I am thinking my amp is going bad, but I am no pro so.............I have no clue. Please help

how would I check the board on the amplifier?

The ground is something I did not checked. I did check the gound cable at the amp but not at the car.

Ok I just checked the ground for the amp and the cap and they are secure. I also made the speaker wiere from the amp to the bos shorter since it was a bit long (don't know if that would affect anything or nto but i did it anyway)

I also have tried to disconect the bass control knob when this happened before with no difference at all. Problem was still there, so I think this will eliminate the knob as the problem.

Can one of the symptoms of a bad amp be attenuation?

I think I am going to re-wire the subs inside the box.  Last time I had this problem I pulled the box out of the car to test it in my home stereo and it did not worked the first time I plugged them. I took   both   the  subs out, tighten everything up(there was no loose cables but I did tighten them up a bit more anyway) and put them back inside and plugged back into my home stereo again and the worked fine. Can the subs be going bad? vibration, making the subs fail? I have them in series/parallel 2-10" DVC 4-ohm