My Gearbox has two converters in it as well.
I was told that they put the additional converter in when they upgraded to the "rock the box" stereo system because of the additional draw of the amp.
On mine, the stereo plays off the 12v just fine - there is a fuse for the stereo that is by the circuit breakers in the underbelly. Mine is also a Kenwood deck and Kenwoood amp.
I have put a Xantrex 1800 watt inverter to run the TV's and such off the batteries - one of the best improvements you can make.