For less than $500, you're going to be looking at the EP125 or maybe the EP175 if you stretch your budget a bit. Axiom subs are very good, but they are (IMHO!) a little bit more expensive than some of the competition's lower-end subs. If neither of those float your boat...
SVS :
25-31 Pci's on sale for $499:
http://www.svsound.com/products-sub-cyl-pcpow3.cfmOr a PB10-NSD for $399:
http://www.svsound.com/products-sub-box-10nsd.cfmAnd HSU's site:
http://www.hsuresearch.com/subwoofers.htmlThe VTF's are often said to offer amazing value in the world of subwoofers. I have thought that if I were to ever set up my M22's in a more critical 2-channel listening system, that I'd probably pick up an STF-1 or -2 to handle the lowest bass that they can't. For ~$350, I know of no less expensive sub that get's as much praise.
However!! I don't know how either company handles shipping to Canada. I think SVS actually does NOT ship to Canada, but uses a distributor.
There, Axiom probably has a very distinct advantage in that they are, for obvious reasons, very Canadian-friendly.
Best of luck!!