You should do some modeling with Winisd. It's extremely helpful when it comes to understanding box size requirements, spl levels at different wattage levels, and comparing different subs.
I have the Crown XLS 1000 and use it for a sealed 18" Dayton HO. I'm currently running bridge, but really just to do so. The output levels running with a single channel are almost too much for my space. (Live in an apartment so 350w IS too much really, but I like bass.) Bridging simply gives me more headroom, and allows me to get the most out of my amp. Building two sealed 15" subs and feeding them each with a channel from your Crown 1000 should be plenty. More than plenty actually, as you would most likely only ever get near 350watts per channel when you're testing or showing off your subs.
So as others have said, download Winisd Alpha and play around with it. Need help figuring out how to get started with it, just come back here and we'll help.