After some careful considreation, I think I have a solution that might help. Did you try pouring juice on the amplifier? Sometimes amplifiers need a some juice because at thier age, they need it to stay healthy and strong. Seriously though, are you sure the amp is getting enough power? You should be running 4 guage wire from the battery and maybe even have a cap (1 farad). Is your battery good? The Motorcrap battery that came with my car was no good, it put out low voltage even when fully charged so I replaced it with a Canadian Tire battery which, although less expensive, worked much better. All of a sudden my car started easier and the stereo sounded better. At low voltage, your amp won't put out as much power as it's rated at. This is just a guess but it might be worth looking into. Don't pour juice on your amp though, I was kidding about that...
------------------
The Honkeytonk Monkey
Jewel Green 96se MTX