Your head unit doesn't put out 45 watts, despite what it may say, adding a 4 channel amp would help, you don't need a cap.
I have a new system, 4 infinity kappas and kappa perfect 12 inch subs. I already have an amp powering the 2 12"s, would it be stupid to get a 4 channel amp and power the rest of my speakers, as the speaker rms is 50watt (or so, may be 70 in the rear?), and my head unit ( an alpine CDE-9852) only tops out at 45 watts. Would it make sense to get a 4 channel amp or am I being stupid? And would I need a cap for this if I did choose to get it?
^^^ yes, that 45 watts is max, its probably around 16 watts rms, get an amp nothing powerful comething like 50 watts rms per chanel, and then just set it to hpf and then the sub amp should be set to lpf and then you will get your music playing louder without distortion
what kind of car? ever consider dropping the rear speakers?
if i were you i'd probably just get a 2 chan amp and leave the rears powered off the deck.