I have a 568/596 running a pair of Martin-Logan SL3's for my main speakers, electrostatics (even hybryds) are not know for their bass output. For movies I have been using a Paragigm Servo-15 but because of intergration issues I had not been relying on the sub for music. Recently I purchased a digital (48k/24) Parametric Equalizer for <$150.00 (US), I was unsure about this unit so I talked the shop owner (music equipment store) into letting me return it within 14 days if it didn't work out. I have not return the EQ, whatever sound degradation it is introducing is more than made up for in the smoothness of the bass. Before installing the EQ (in line with the sub only)I used an audio function generator (set for sinewave) to plot the in-room response of the sub, from the deepest null to the highest peak was 27dB! It was no small wonder why I could not get the sub to sound good for music and it sounded boomy for movies. I am not quite done tinkering with the EQ yet, but I can say this, I am now setting my mains to small and crossing over at 65Hz. I am very pleased with how the sub is now intergrated into the system. As a bonus, since the response is now flatter, the sub can now be turned up for movies without sounding boomy or one note-ish. This added sub output really gives the movies more impact.
If you are happy with the sound of your KEF's (excluding lower bass) I would think about a sub, having the couch shake while watching a movie is truly a wonderful thing.

Steve