Amplifier Problem

 I have two HiFonics Zeus Z200 (Supposedly 200 WRMS) on two 12" Kicker comp VR dual 2ohm voice coil. I have each sub's voice coils wired in series to produce one 4ohm load. Each sub has its own Zeus Z200 mono to supposedly produce 200WRMS. So the amps are getting a mono 4ohm load. Anyways, after about 15 minutes of playing whether at a low volume or high volume, the bass from the subs will just cut down to about half. The amps are not getting warm. They are wired correctly. Here is the kicker, if I turn the volume up really loud to the point of distortion for just a secoond, the bass kicks back up to what it was for another few minutes. I know I am probably underpowering these subs, but would that cause such a problem? 

Also, I am thinking about replacing the amps. Which one...a MTX6500D or a JBL BP600.1? I have heard that both of these amps really slam Kicker 12" VR, but before I replace the amps, I want to know that they are the problem. This would not be a sub problem would it? Head unit has been replaced already.

has this always happened, or is it a recent development? try running one sub and one amp at a time...you may want to try a different pre-out set on the head unit if you have that option...

it is hard to say what is happening...make sure your grounds are really good, and maybe test your alternator and battery...I once had a similar problem, and it was actually the decks pre outs that were causing the problem...

   I changed the deck a month ago. \I had the problem before and after that.  Now I have changed the amps. I am now only running one BP 600.1. It hits hard , but the problem is still there. The bass cuts in half after a few minutes. The amp is not getting warm. The protection lights are not coming on. Could this be a sub problem? Is there anything that would cause a sub to suddenly produce only half  for a few minutes. Usually after a few minutes, it will start hitting hard again.

did you try trouble shooting the rca-jacks?