kelloo, u may need a preamp once u've upgraded yr sound system to multiple amplifiers. now currently i believe u are running all stock right? yr deh2650 cd player has only 1 set of preouts for a single amplifier, which will be used for the sub u intend to get. if next time u want component set, u might have to get a preamp to connect another amplifier. otherwise, u can get a HU with more preouts. a cheaper alternative(ofcos, sacrifice sound reproduction) is by using a Y-splitter, which will divide yr 1 preout into 2.
at the mean time, if u intend to run yr stock speakers with only 1 amp and sub, no problem, no need preamp.
let me noe when yr fren wan to contact me la.
cheers