I've had this sub for a few weeks now. BK are OEM suppliers for several subwoofer manufacturers, and I bought this sub after the recommendations of users here and on avforums - thanks for Hazzaman for his feedback. It's the first sub I have owned - unless you count a crappy Logitech 2.1 PC speaker system with it's one note "sub" - so it's hard to review without having any basis of comparison.http://www.bkelec.com/HiFi/Sub_Woofers/XLS200-DF.htm
My first concern would be that it would provide a noticeably artificial bump at lower frequencies. No problems here, with very minor tweaking - setting the cut off at around 60Hz, admittedly above the supposed 45Hz rolloff of my Dynaudios, and setting it to about 30-40% power, it blends quite seamlessly with the lower bass from the main speakers. Unless there is a lot of energy in the lower frequencies I can't often hear the sub as separate from the Dynaudios, rather it fills them out with a little extra bass extension. Besides providing frequencies that are not there, it seems to provide a fuller sound towards lower notes that aren't really in it's range, such as double bass hits.
To my ears it has excellent control and definition, when there are defined low bass notes, for example with Roots Manuva or Boards Of Canada, it is audibly playing clearly different notes rather than going "WHAH WHAH WHOAH". Although you can crank it up a notch and it does that too
Subs are notoriously difficult to place without using a sound meter, and to be honest I've just whacked mine against the back wall and haven't done any proper setup. I played a stepped sine wave through it and there are definite room issues, but the real nasties are actually out of it's frequency range and at the bottom of the Dynaudios. I will get around to nerding er looking at my room acoustics but I'm too busy playing with wires and things at the moment.
What this sub doesn't do is provide real bass slam - it's not a Funktion One sound system and it's not really a movie sub either. For that I gather you'd be better off with the XLS300/Monolith in the BK range. It has a gentle rolloff which I think contributes to it's natural sound with music and due to it's size - about a foot in each dimension - it doesn't have real low bass rumble. For that BK have a new XLS400 sub which is supposed to have extra bass extension whilst still sounding natural with music. But unless you really want real junglist slam or have a very big room I think the XLS200 is plenty. Highly recommended, especially at it's £300 pricepoint.