head unit preamps of channels?

I have a project car that I'm going to be installing a double din DVD player in. I'm just running the front and rear speaker off of the radio (no amp), but I'm also going to be running two subs off of one amp. I'm getting a little confused with some of the head units I'm looking at. Some say 2 channel, others say 4 channel, some are 6 channel. For example, one unit I like is the Pioneer AVH-P2300DVD. Some sites say that it has a subwoofer preamp, others say it doesn't. Does the head unit have to have subwoofer preamps? Or can I use a 2 or 4 channel? If I get a 4 channel, does that mean I have use a 4 channel amp?

The 2300 does have subwoofer output.  With it, you can easily adjust level and crossover point.