Xplod is good. CDT Audio and JL Audio also make great subs. I've got a CDT Audio 10" sub mounted in the rear-door spare tire space, in an enclosure custom-built for me by Shawn from this forum (username = psychos2). I love it!
I bought a 400 watt Blaupunkt subwoofer amp from Fry's, on sale, and I'm completely happy with the sound. I didn't change my stock speakers; just added the amp and the subwoofer. The system sounds a million times better now.
OEM roof rack, Thule Xpedition 696 rack, Rancho RS9000 shocks, OME 913 coil springs, 3" lift, American Racing Teflon Chambers wheels, Yokohama Geolandar HT/S G051 tires (275/70/16), Energy Suspension greaseable bushings, steel-braided Kevlar-reinforced brake lines, WeatherTech wind deflectors, Malibu 18w driving lights, Plasma SuperWhite 100w headlights, NGK Iridium plugs, K&N drop-in air filter, Optima Red Top, Psychos2 rear-door subwoofer enclosure w/CDT Audio 10" sub, Blaupunkt subwoofer amp, iPod Video 80GB, Kenwood KVT-617 DVD head unit w/7" motorized touch-screen, Kenwood GPS HD navigation module, Kenwood iPod adapter, Kenwood back-up camera, Sirius radio, Flowmaster Extreme Off-Road muffler (black diamond-plate steel) w/ carbon fiber tip, Ford Taurus OEM cup-holder/coin tray (single-DIN/lower bay), personalized license plate "ORBITAL", 3M window tint, debadged, removed rear seat, custom 'radiation symbol' side magnetic signs, VX Concepts front skid plate