The only place a cap would help you with that system is right next to the amp that is powering the subwoofers. It should be within 6 inches of that amp. What the cap does... it will cause less voltage dips on hard hitting bass. (if your amp can supply that) Usually alpine amps are very efficient.
The advantages... -some of these are a stretch...
more consistent voltage through out the entire car. (you might notice your headlights stop dimming)
better running car... ie more voltage to the coil...
longer life on the alternator...
Better sounding stereo... ie if your sub amp was causing a voltage drain that could cause less voltaged to the head unit... which in turn raises your Total Harmonic Distortion ie THD...
I used to run 2 accuvolt 1400 .. I had them set to 14.4 volts..
it made a world of difference because the amps gained about 20% power rather then running them at 11.5 - 12.5 volts... anyway
I digress...
I really wouldn't waste money on a cap though if I was you...
I would by a battery isolator and run a secondary battery for your radio.
ones like below... they are out of stock but I am to lazy to do more searching... you get the idea
http://www.donrowe.com/battery_isolator/isolator.html
FWIW..
I was running 12 12" JBL subs in my 67bug.
I had a wall built.
total specs:
3 x JBL 1200.1 mono amps
1 infinity amp 4 channel.. dont remember exact specs..
12 JBL GTO series dual voice coil subs.
2 1500 Jacobs Electronics Accuvolts
1 battery isolator.
1 custom built alternator
2 yellowtop optima batteries..
man I miss my car
