Audio Systems: car audio system problems, sony xplod, full tilt

Question
i have had my system for about 3 months now and i biult it myself, (head unit: indash jensen, 2 sony xplod 1000watt each amps, 2 10 inch audiobahn peak 1000watt subs and 1 4-farad capacitor. each sub is powered by a seperate amp so they dont share power. my problem is that recently for some reason theyve lost power in the sound. wen both subs are playing, they sound muffled and strianed and it sounds like they are only putting out 10% power each. but wen i unplug one of my amps so that only 1 sub plays, the sound that one sub puts out is alot stronger but its still only like 40% of the power it was putting out before and wen i turn up the volume, the sub flaps distortedly like its being maxed out, but i no it would never max out at where its maxing out at. is there sumthing wrong with the way its wired up? or is sumthing wrong with my headunit, amps or subs??

Answer
Something is wrong - for sure.  But, I think it is an easy fix.

I want to focus on two possibilities:

ONE:  That power (the 12v lines) are not getting full tilt to the amps from the battery system.

TWO:  The subwoofers are losing their surrounds and even though power is getting into them,  the acoustic power is not getting out.

SOLUTION:  
ONE:  Go to Walmart and purchase their $9.83 vom - if you don't already have one.  This is a very useful instrument for checking out car systems.
TWO:  Check the line voltage feeding the amps.  If they are not getting full 12v (actually, a fully charged battery should give you closer to 13 - 13.5 volts.  Check the voltage when the amps are not putting out sound and when they are so you can see if the voltage drains down when the sound volume goes up.  (Put the red test lead on the 12v line coming from the battery and the black test lead of the vom to any good metal surface on the car.)  Check the voltage both at the battery AND at the amp.

If voltage is draining down you must check for bad fuses and bad grounds.  Bad grounds are famous for causing this kind of problem.

THREE:  Checking the subs is easy; just look at the surrounds attached to the cones and see if they have rotted out, gotten torn or in any way damaged and letting air come around the cone.

Let me know if you need more ideas.
cleggsan