With a space like yours demos are pretty much useless. The top contenders in the 18" class are the Velodyne, the Funk 18.0, and the Funk 18.3. All look good, are built well, and the Velodyne competes with the 18.0, not the 18.3. It's not clear how the DD18 compares with the 18.0, no one has reviewed the 18.0 yet, though the Velodyne has a remote, selectable programmable profiles for different usages, and IMO a better integrated look with the grill on, if any of that matters to you. The Funks have a much wider selection of finishes than the Velodyne, which has two. I suspect that performance of the two products are comparable.
You might find Velodyne dealers with DD15 Pluses in their showroom. They look identical to the DD18 Plus, just smaller, so you can get a feel for quality, finishes, and features. I've never seen a Funk sub either, but there are owners on this forum that'll swear to awesome quality and results.
The Funk 18.3 is much bigger, especially taller, so you'd have to be able to tolerate what are essentially two additional tower-size cabinets in your room. Funk says the 18.3 has similar output to two 18.0s below 40Hz, so it might be your best choice for rocking that massive room. At least in the US the street price of the DD18 Plus is about the same as the 18.3, the 18.0 is considerably less. I'm not sure how Velodyne prices in Canada, or what dealer discounting policies are.
If it were me choosing, I'd go with two Funk 18.3s.