If by preamp outs or RCA outs you mean off the empeg, there are two pair of outputs off the sled. There are only two off the chassis itself. The sled also supports an additional pair of inputs. It is unlikely you will find a head unit with two pair of auxiliary inputs on the rear that would operate in unison. Most frequently the head units will support front/rear fading and can be driven from a single external input source at a time.

Typically they are used in pairs right (red) and left (white) and then those pairs are grouped according to their use (front/rear, Aux In/Line Out, etc.) Again, the FAQ pretty much covers it. See the section "How do I run a subwoofer if the player doesn't have a subwoofer output?" for a few diagrams.

Tim