Thanks for all the replies, I'm sure you guys know about this stuff a lot better than I do but I'd just like to clarify a few things:
1) The box for this sub isn't small, it's roughly the same size as a fairly high-end home theatre subwoofer box that I used to own. Also it weighs a ton - there's a pretty hefty magnet base on the driver
2) Home theatre isn't my target application, I'm planning to use it for occasional (once in a blue moon when the family are away) parties, so it'll be music rather than movies
3) The JL box is effing powerful - at full tilt it makes the bones in my ears vibrate, and before it was dropped into the car I had it demo'd in the shop which is roughly equivalent in size to my living room and it sounded pretty good there too.
4) The monoblock is 1000W but the sub only draws 600W max I think, so I probably only need a 750W PSU to drive it effectively.
Given how awesome it sounds in the car and how expensive the rig was just for the sub box and the monoblock, I'm still very skeptical that I can get similar performance indoors for only $100 or so.. but I'll defer to people who know better