i'd also look at where you are putting the sony amp...it needs a lot of air space. the circuitry they use overheats pretty quick...especially if it is powering subs at any ohm load below 4 ohms. I'd take RF over JL and a fosgate amp rated at 900w is gonna give a little more power than the Kenwood i seen you mentioned at 1000 w...the circuitry they use is an H class type...it switches to 2 different pipelines when the amp heats up...keeping it cooler over all. Cooler amp more constant power. And a capacitor stores energy until you need it...like during hard bass hits. It keeps the strain off the alternator.
Now with a capacitor make sure you have it wired correctly, i've been shocked by mine...it isn't pleasent... PS. i can back up the H class design because my amp is based on it..A Diamond Audio 1000 watt mono. i have it wired at 1 ohm and it doesn't get hot at all. My cap is a Boss cap but the trick with these are that they are representative. one company makes them..and a lot throw the badges on them.. BOSS, Stinger, fosgate, JL, and Tusami are all the same. Just pick which one you want.... SORRY FOR THE LONG POST I HOPE I HELPED YOU OUT

Umm, JL is not fosgate at all. Totally different stuff. Much more performance oriented. MAYBE for the capacitor but that'* it.

RF quality is average these days, and they aren't underated, like they used to be. Kenwood amps are decent, and JL has many features, one keeps the same power output at different ohms. etc.

yeah..i only meant the capacitors...oh i know RF and JL amps and subs aren't the same..lol.. i prefer Kove Subs myself though