Okay a deck close to $100 bucks. That's gonna be a tough one. Check out ebay maybe.<br />There are a couple of reasons for more than one set of preamp outputs on a headunit. Ease of adding additional amps for mids and highs. If using both sets usually you will have the ability to use the fader on the head unit. Some decks have a third set for a sub output. Does it make a difference ? If you want to upgrade later yes. If you only plan on using one amp no. I doubt you will need another amp for your subs. I seriously doubt you will need a cap. A cap doesn't act like a battery. A cap will only offer very,very short bursts of power. About the only thing a cap will help you with is to keep your lights from dimming. I don't think you will have dimming problems.
5 % equipment
90 % install