New HU
Keep in mind though the number of channel outs with a stock system will not make any difference since the Monsoon uses speaker level output from the unit not RCA's. Unless you plan on using an aftermarket amp and ditching the Monsoon amp, RCA outputs become irrelavant beyond maybe adding a sub down the road or something.
Another thing people get caught up with when buying aftermarket for the Monsoon is output wattage. This is also not worth worrying about because they Monsoon amp is what dictates wattage output, the signal from the HU is merely a signal. The benefit of aftermarket is that the signal from aftermarket is generally a much cleaner, processed signal so that is where the sound improvement comes from. It also allows for more volume not because there is more power but because the signal is cleaner and distorts less at high volume so the power that is there has more useable range.

