subs get quieter for no reason

Hey first time poster in this forum.  I have an issue that has been driving me nuts.  First off, I will tell you a bit about the equipment involved.  I have a Pioneer DEH-P7400MP deck with two Pioneer twelves and two smaller Pioneer amps.  Not sure of the models on the amps and subs, but I have 1 amp running each sub with the power bridged on each amp and sub independently.  Everything is hooked up fine.  I have checked and rechecked my connections, and I can't find a problem anywhere.  I have been running this system the same way for about three years and had no problems until now.  The problem is this...when I am listening to anything, the bass in the subs will get quieter for no apparent reason.  It will come back to a normal level again, and then it might go back down a minute or an hour later.  There is no rhyme or reason to it.  It may work fine for a stretch, and then not work right at all for a long period of time.  The most important thing to note, is that the subs are not cutting out at all.  They just die down and then go back up again.  I suspect its the preouts on my deck, but I am not sure at all.  I just don't want to replace the deck, and it doesn't help, or replace the amps or subs and then find everything works great after I have replaced everything.  Any assistance would be much appreciated.  I have installed systems professionally for about 8 years now, but this one has me totaly stummped.

Thanks for a quick response.  The vehicle is an 05 mitsubishi outlander.  So the ground on the battery is almost new, and the ground for the amp was in a remarkable location as far as good solid metal and distance from the amps is concerned.  Any other thoughts?  I noticed someone had posted a response in another thread about overloading your turn on lead if you have more than one amp hooked to it?  I to have this configuration.  Any response to that theory?  Thanks again.

I usually put a relay on the turn on lead if I'm connecting more than 2 amps, but that's not your problem if your subs continue playing (the amp is still powered on).  I'd probably swap the RCA's to another preout first to determine if that is the problem.  If this annoying lack of bass goes away, you've found the problem.My ex once told me I have a perfect face for radio.

Thanks Again.  I only have one set of rear preouts on the deck though.  Any other things you recommend I might try?  Would a short in the RCA cables make this happen?  That is another thought that had crossed my mind. 

Yeah, it could.  Check to see if there's a pinch anywhere on the RCA's.  If you unplug both ends on them, you should not be able to read anything across the two conductors- it should read open.  If you read a high resistance (aside from Mohms which is usually an open), then that could possibly cause what you're describing. My ex once told me I have a perfect face for radio.

I will have to get out my MM and do that.  The only other thing that I noticed that struck me as strange was the following...I have my power cable fused at the battery and fused in a distribution box in the back.  The fuse by the battery is a glass type fuse. I noticed that the conductor in the fuse was bowed down like it had gotten hot enough to flex, but not enough to break it.  The conductor in still in tact though.

Replace the fuse if it doesn't look the same as a new one does.  Be sure the fuse if the right rating for the wire in question.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

cold solder joint, loose connection, failing component withing hu or amp? if out of warranty consider reflowing suspicious solder joints, connections.

I replaced the fuses, checked my rca cables for shorts, and still the same thing.  This is driving me nuts.  It seems to me that both subs are doing the same thing, so I suspect my preouts are going bad, but I am not for sure.  I think I am gonna have to replace the hu and see what happens.  Any other thoughts would be greatly appreciated, thanks again.