Another vote for BK. I've got the XLS300-FF and it's a great sub, goes low enough to pump vibrations through you. I went for a Grade "B" from their eBay store and it arrived completely mint for £338.86 + £10 postage.
Mine is a similar size room, 21.5m², with the volume being 64.5m³ (3m ceilings) and I'm only running it at around half power. I was also considering the SVS SB-1000 Pro, which PT have in at a discounted £549 at the moment, but it's hard to beat the XLS300 where you can save £200.