I am just going to chime in, please take my advice for what it is, some random guy on the internet:
1) Noble 100 is absolutely right, the more subs, up to 4 (more than 4 provide marginal improvements), the better off you are in terms of smoothing out response and improving sound quality. There is no amount of DSP that can make up for doing this acoustically.
2) Most room treatments provide minimal help for frequencies less than 100hz. To effectively work, they must be large, and to remove large peaks must have a very narrow bandwidth (look up a Helmholtz Resonator). GIK has excellent room treatments for reasonable prices and will, if you provide your room information, make suggestions.
3) This a controversial point, but in my experience high passing your main speakers makes a big difference, as it removes their trying to reproduce bass frequencies that increase distortion of the bass driver in your main speakers. For some reason, people will argue against this, but I don't see them arguing you should remove the cross-over between your woofer and tweeter?!!!
In the case of your KEF's, that driver goes well into the midrange. I also agree with Richard Vandersteen, that to fully integrate a subwoofer, your mains should have solid extension one octave below the cutoff frequency. Your KEFs are close enough that using a high pass with them at 80 hz will work.
4) The vast majority of what we perceive as "slow" or "bloated" bass are room nodes and slow room decay. Unless so poorly designed (generally a vented enclosure tuned to get maximum output) that transient response (really group delay) is so high it becomes audible. Virtually no well designed home subwoofers suffer from this. (As a note, most people perceive vented subs as tubby, because they have greater output: until recently, many, even high end, sealed subwoofers had a roll off starting in the 40hz range, now fixed by DSP).
My two cents, get two or three less expensive subwoofers, carefully place them, and cross over your mains. Rythmik offers the excellent L12 (white for $619 if you want that color) or LV12F, SVS the SB-1000 or PB-100, HSU the VTF-2 or ULS 15, or the RSL Speedwoofer 10s for $399, which gets you 3 for $1200.
