I've been trying to change my signature without success, I now have an SVS PC-12 Plus subwoofer (with which I am very satisfied) in the parlour and two Tannoy TS12 subwoofers in the kitchen.
On page 62, (in a group test) and pages 74-76 (in Temptations) of the May 2012 issue of What Hi-Fi the £1,300 Jamo D600 subwoofer is mentioned. It has a 15" driver. Quotes from the two reviews about include "It's fast and accurate, and capable of truly sofa-shuddering bass", "If you want cinema-level bass quality in your living room, the D600 will not disappoint" and "placing huge, impossibly muscular thunderclaps of bass into even the wide areas of your room".
Your room is quite large, 50% longer than my parlour. Before buying my SVS sub in March I contacted their dealer in Norway, LSound, at that time SVS did not have a dealer in the UK, though they do now, Frank Harvey Hi Fi of Coventry. LSound advised me that for my size of room (18'2" by 14'3" by 10'6") the SVS PC-12 Plus was certainly adequate. But for your size of room, perhaps the top SVS model, the PC-13 Ultra might be suitable? Frank Harvey’s prices are about £100-200 cheaper than I had to pay getting mine from Norway! The PC-13 Ultra costs about £1600 and has a 13 1/2" driver. The Norway dealer has several reviews of it, one of which is from Home Cinema Choice magazine four years ago, May 2008 here
http://www.lsound.eu/userfiles/file/Reviews/SVS/PC13-Ultra/PC13-Ultra%20HCC.pdf . The verdict was "Mind-bendingly deep yet tight, melodic and controlled bass." That is of the previous incarnation which used a 750W BASH amplifier, the current model uses a 1000W Sledge amplifier and is better than its predecessor. SVS also do the box rather than cylinder model, the PB-13 Ultra.
While you are speaking to Nick at the Home Cinema Centre you might ask him about his second-hand REL Gibraltar G1 subwoofer. That has a 12" driver, costs about £3000 new, weighs 49kg, the -6dB point is at 15Hz and was reviewed in the June 2012 issue of Home Cinema Choice magazine, with the conclusion “Ground-rippling lows with intense grip”.