The same import site also has the UM18-22, and RSS480HO (18").
However, I fear that any 18" woofer / box is going to fail the WAF. I am being optimistic with the 15" version. Haha.
But first thing first, is I'll see if anyone is willing to build me a box here (for a reasonable cost).
(Actually, I wonder if I can persuade the wife the subwoofer boxes can double up as side tables next to the couch?