Amplifier problem?

First off, Im running a 15in Kicker Solo-Baric L7 with a Rockford Fosgate P5002 Punch Amp and an Alpine P4900IB headunit. Bought the Amp used about 3 months ago and it's been hitting HARD the whole time until today.
 
Ok, so for some reason my amp is not working right. I had my volume pretty damn low (at 5) because I was talking to my brother. All of a sudden the sub stopped working, so I thought one of the wires came loose. 
 
Checked the power, ground, remote, RCA's, wires that go to the box, everything. 
 
The amp still powers on, but when i plug it into my box, the sub just fully extends out and stays there. I dont hear any music going through it, it just fully extends out. So I plugged in another sub and it does the same thing, just extends fully out. So I crossed the + and - of the sub and it goes in fully and stays. 
 
Hook it back up the right way again, sub fully extends out. Unplug the RCA's, same thing, no effect at all. So then I unplug and plug back in the sub a few times, and the sub starts working again, I can hear music through it and it bumps as it should (the volume is NOT high at all, I put it to 5, just high enough for me to hear the sub working), but as soon as I turn the volume up a little higher it stops working again, and sends the Amp into safe mode for a few seconds. When the Amp goes back into regular mode, the sub does nothing, doesnt extend out or anything. I then unplug the sub, then plug it back in, and the same thing, it seems to work fine, but as soon as I turn the volume up a notch it kicks the amp back into safe mode.
 
If I turn the sub off from the head unit, so that NO signal is going to the amp, if I plug the sub in, it fully extends out again.
 
Does this sound like the amp is malfunctioning? It just keeps sending constant power top the sub, even when I turn the Subwoofer off through the head unit, then when I turn the sub back on, itll work until I turn the volume up a click, then itll make the amp go into safe mode again.
 
Keep in mind its NOT the sub, because I plugged in a spare sub I had and it still did the same exact thing. Took out the head unit and checked the connections. Checked the Amps chassis ground, fuses, power wire, remote wire, RCA's, everything. i even took apart the amp to see if anything burned out but everything looked normal. Tapped on the board with my finger all around it to see if anything was loose but nothing....
 
Im Stumped.

if your sub is a dvc type check your jumpers on the terminals to see if they are shorting on each other.  most fosgate amps only go into protect if it senses a short.  but then again it can be the amp since you said your tried it with a different sub.  also check the rca terminals on the amp if you see cracks in the joints where it connects to the circuit board that can be your problem too.

So the subwoofer fully extends even when there are no RCA wires hooked up?

sounds like you a short between your sub and amp...check your connections....or run new speaker wire....to make sure... or if you have a multimeter...disconnect your speaker wires from the amp and measure the resistance from the "-" to "+".. .... should be whatever your speaker is (4,8,2,1 ohm) depending what your set up is.....if its 0 you have a short.

Took the amp to a friend of mine. Turns out the problem is a shorted output transistor. In otherwords, send it to get fixed or buy a new one. In my case it's time for an upgrade anyways...