I don't really know a lot about head units. 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.
if your head unit is rad - it will allow you to custom talor the sounds that you send to each channel of your amp - cool if your amp does not have good tuning features.
lets say you want to run your amp to your front door speakers / and subwoofer
you can tune channels 1 and 2 ( left and right ) to the front speakers to block out all the bass. bass does not play well from door speakers. so you can then feed the door speakers more power with them sounding better as they wont distort from the low frequencys.
same for the sub
you can tune out all the higher frequencys and just send low frequency to the two subwoofer channels and then your sub will play clean down low with no muffled high or mids trying to play thru its cone.
another aspect to look out for is the rca output voltages. anything less than 4 volts coming out of the rca from the headunit is basicly concidered low
and you might have tuning issues with your amp if you suffer from low rca output voltage
4 preouts is a minimum. It would be front & rear pairs. Some (most) headunits will allow the rear preouts to be defined as non-fading sub preouts, but you lose rear channels. The 6 preout models have dedicated sub outputs.