Rpm dropping?

There are many reasons why your RPM could drop when the bass hits, and none of them are good.

I would assume that your electrical system isn't capable of keeping up with your stereo.  When the bass hits, the voltage drops.  If the voltage drops far enough certain electrical systems in the car could cut out, including but not limited to your ignition system, your fuel injectors, and your fuel pump.  It is also possible that if the voltage isn't stabe the ECU could turn itself off to protect itself from the unstable power.

In my old car ('94 Cavalier) when the bass hit my ABS system would fault and would stay off until I restarted the car.  I did some upgrades (optima deep cycle battery and a 1F cap) and the problem went away.

It depends on your situation.  I wasn't pushing 20,000 watts, I had an 800 watt RMS (Kicker ZR600) amp on an old stock battery.  My alternator was technically making enough power for my application, but my tired battery just wasn't able to keep up.

I'm assuming in the original posters situation a new battery isn't going to solve the problem as it sounds like his situation is much more serious then mine was.