I don't know if Ian is still building boxes, but he might be. If you are going Infinity get the Kappa Perfects and not the Reference series. Huge improvement and not that much more expensive.

How much are you looking to spend? Why are you looking at 2 subs? You would be best with one nice 10" or 12" IMO. 15" isn't necessary and I think a single sub will hit harder and cleaner off of your amp.