Everybody's gotta start small. The box was most likely designed to be used with the sealed section, like I said before a bandpass is part sealed, part ported. I've never built or even owned one though, so I'm by no means an expert. Basically any sub set-up will sound loud at first, but you'll get used to the Kickers very quickly, they are really not that loud,
You could probably get a new set-up pretty cheaply, a Single SA-15 in the proper box on that Treo @ 2 ohms would definitely slam harder than the comps. And if your local Craigslist is anything like mine, you could get most of the money back out of the comps on there.
Is sa-15 a 15? If so, are they 4 ohms or are 15 2 ohms?
You do need to pick up a Digital Multimeter, it is an essential tool for setting up amps, and even having a proper set-up can't even prevent clipping, you've got to know what you're doing, base rules are never go up past 80% volume on the headunit, and set your amp's gain to slightly under the rated output there.
Your sony headunit is definitely a problem too, don't cheap out on the headunit, it is the base of the system, I've had 500$ systems where 250$ of it was the headunit.