Ok, we sort of have one design requirement...max of 19 ft^3...is that internal gross or net or external volume?
For the A500, while you can limit yourself to 8 ohm drivers for bridged operation, you're probably better off looking for a dual 4 ohm coil or using dual 4 ohm drivers. Same power delivered from the amp, more driver selection, and the amp will probably be happier not running bridged.
How about budget? Using an SPL meter, how loud do you listen? What are your main speakers, rest of system? How far will you be seated from the sub? How big is the room?
Right now this is just a modelling exercise which you should be able to easily do yourself. Download WinISD Pro or Unibox and have at it. Be sure to read WinISD's help files on how to enter driver parameters.
Some recommended reading:
http://ldsg.snippets.org/idx.php
WinISD help/technical articles contained in the applications help section
http://www.diysubwoofers.org/
Limited to the A500...for the money, you're probably not going to beat PE's DVC15. On sale for $99/each with free shipping right now. Dual 8 ohm coils, wire in parallel for 4 ohm/driver. Get two, build a box of 12-14 ft^3 net internal tuned to 15-20 hz. Duals also gives you the option of building 2 boxes to help even out room response.
Another relatively inexpensive option, two TC Sounds db500-12s in 10ish ft^3 tuned 17-20hz. Probably very close to being an underpowered SVS Plus/2, but you're offsetting the lack of power somewhat by using a larger box. Not as much output as the dual DVC15s, but a flatter curve.
-Brent