I've got kind of a dumb question regarding head unit and pre amp pairs

do i need three pairs of pre-amp outputs if i plan on adding an amp & sub in the future? i have my eye on a Pioneer DEH-P6700MP, but it only has 2 pre-amp outputs. the Pioneer DEH-P7700MP has three pairs, but i'm not a big fan of the single **** design.

also, does anybody here have any experience with either of these head units?

 

You only need two if you want to add one amp and a sub.