Since you are located in Canada and your budget is 400 - 500 dollar range, I'd suggest the SDX15 for $259 shipped
http://www.funkywaves.net/catalog.cfm?item=sdx_15
or the Mach 5 Audio IXL-15 for $230 shipped.
http://www.mach5audio.com/product_in...bcfaf02d824f42
Both subs work well in 5 cubic feet tuned to 20hz.
Isobaric loading is when two woofers are coupled together by a short enclosure which is only long enough to prevent the two woofers from striking one another. The woofers can be mounted face to face, back to back or front to back. It is important however to have the woofers firing in phase with one another. Vas is halved and there is a 3db loss in efficiency. Isobaric is also known as push-pull, push-push, compound, and clamshell.
Changing the high pass filter on a Bash 300 is done by changing 2 resistors. A chart is provided with the various combinations.
http://www.partsexpress.com/pdf/300-750.pdf