My advice, Tom: buy a passive sub driven by an outboard amp. I can't tell you how many subs I killed before I got wise to that. It seems to me that even some expensive powered subs use pretty cheaply made plate amps. Think about the environment a plate amp has to survive in- it's crammed into a compact box with a lot of heat & subjected to constant vibration. It's a wonder any of them last. Just look at the warranty of a typical powered sub. That will tell you how long the manufacture thinks the thing can survive.
With an outboard amp, you won't have this problem. The amp can sit on your rack, cool as a cucumber. Plus it won't be shaken to death by the action of the driver. Another nice bonus is that it's easy to adjust the volume. Just why is it that on 95% of powered subs, the %^# volume control is on the back of the sub?
Some good brands that make passive subs: Hsu, VMPS, Adire Audio, SVS & Selah Audio. I'm sure there's brands I'm forgetting, but the ones I've listed here are some of the stoutest subs in the world by sheer SPL. S&V magazine measurements of the Adire Sadhara (sp?) showed tremendous output, and you'll find scores of great reviews for the Hsu & SVS subs (their volume at very loooowwww freqs is staggering). Selah makes a sealed sub called the "Whomp" that's extremely tight & musical, while VMPS uses passive radiators and claims > 110dB @ 14 hz!
Good candidates for a sub amp: used Adcom GFA-555 (can usually pick up for $350-400), new amps by Crown, Samson, Carver & QSC. Pro amps work very well for subs; they usually have volume controls, heavy duty protection circuits, hi powered cooling & they're usually stable into very low impedance loads (often down to 1 ohm). They're also fairly inexpensive.
BTW, these guys are all direct-sales, although they have a few dealers. All of them offer best-in-class customer service, and if by some incredibly bad luck you manage to blow them up, you can usually get just the driver & replace it "in the field."
Just my $.02 worth.