You could get a JL E-sub e-110, which has a high pass filter to send back to the mains. That would increase dynamics and handling capacity for the KEFs and the JL would do the heavy lifting to pressurize a larger room. Much less money than the Reference Series floorstanders, and much more flexibility for room size and placement.
http://www.jlaudio.com/home-audio-powered-subwoofers-e-sub
http://www.jlaudio.com/home-audio-powered-subwoofers-e-sub