I'm assuming you want an active sub (with an internal amp, and a power cord that provides it's own power)? Or do you have a good reciever (or separate amplifier) to provide power for a passive sub? I'll only be linking to active subs, just in case.
No incredible deals on the sites you linked.
Soundsaround's website is
here. It isn't all that well designed, nor is it very functional, unfortunately. But here's the good news: they have sales. There was a one day sale yesterday (Friday) with some pretty decent deals. I'm sure the next one isn't far off. Also, they appear to be the kind of place you could break out haggling at, or maybe pick up a demo, open box, b stock, or refurbished item at a heavy discount (if you like that idea). I might drop in my nearest location tomorrow if I were you, see if you can't snag a great deal, and regroup if you aren't totally impressed. You can always wait for the next sale and be the first to jump on it.
As for Visions and Future Shop... they both carry some passable subs. Certainly the brand names to look out for at these stores are Velodyne and KEF. Not sure they'll fit your budget, even after a good haggle, but IMO it's worth a try.
After that, there's one that stood out from each store at a list price within your budget. These two subs are somewhat similar...
Energy ESW-C10 10" Down-firing Subwoofer ::: $350 @ Future Shop
JBL ES150P 10" Front-firing Subwoofer ::: $400 @ Visions
Neither of these brands are exactly renowned for their subs, but these units are capable of delivering the kind of bass output you're looking for.
Energy was a 100% Canadian owned and operated brand until 2006, when Klipsch bought them out (
see article). I wouldn't be worried about buying a Klipsch-influenced product in the Energy ESW-C10, though. Klipsch wants to retain it's loyal Energy fan base, and so Klipsch's engineers likely had very little (if any) direct influence on the design of the 2006-'08 Energy products. They're still separate entities in a sense.
JBL is well known for their professional series loudspeakers and monitors, but much less so for their consumer-grade product. Like Energy, they have a good reputation on the whole.
This would pretty much comes down to aesthetics/price since the specs are so similar. That is, unless you come across a smokin' deal you just can't pass up.