What is the difference between having a head unit with 2 preamp outputs vs 4? I'm running the head unit to a 4 channel amp if that matters.

 

4 makes it easier to run 2 amps

so if you plan on ever running a designated subwoofer amp on top of a 4 channel amp for the mids and highs, then get a 4 preamp

 

Why stop at 4 when you can go 6? I've always had 6 channel pre outs. They work great if you want to really control the sound coming from all your front and rear speakers and subs.



4 is definitely sufficient for most folks. I'd steer clear of 2. Just no where to expand if you ever feel the need.

 

If theres only 2, and you want to add multiple amps, you'll need a 3 way crossover, like these



http://www.sonicelectronix.com/cat_i60_crossovers.html