ok well a head unit is 200w total, thats 4 channels at 50w (max) in theory yes they are underpowered alot but theres alot to take into account than just power, you will find its still a great match so dont stress.
the higher voltage of pre out is a big helper for the amp, gives the amp a stronger signal to work off, better sound/more efficent. a mono block will have the one RCA input (pair RED/WHITE)
and 3 pairs will be better in the long run for running multiple amplifiers.
if your paying around $300 or so for a head unit thats a pretty good chance that your going to get 3x RCA outputs AT 4-5v