the advantage of 3 outputs is so you can control the output of the sub. Most head units with 3 pre-outs have the ability to turn up, down, off and on the sub. also to change the frequency sent to the sub. if you don't care about that, then go for the 2 sets and use the output of the 4 channel amp (if it has one) to send a signal to the sub amp. Good Luck