Sounds like you're really after a home theater sub instead of a music sub. If you still want to use your Kef 4S woofers for bass, set them as "LARGE", so that the general 2-channel bass frequencies are still sent to the Kef speakers. Then just look for a good high-power / long throw / stiff cone subwoofer. The extra bass effects (such as explosions, etc.) are generally produced on the ".1" track of the movies and they would not generally be sent to your main speakers unless you didn't have a sub. At this point, it doesn't really matter to try to match the tonal voice of the Kef speakers, since it is all sub 80hz effects anyways. For general full range sound (music and general move tones), they will still be sent to the Kefs, so no blending would occur with the sub.
In need of some advice on a matching a sub
Hi guys new to all this just wondering if anyone could help me out, im currently running a 5.1 (without sub) system im using kef reference model 4s for fronts model 1s for rears and a 200c center speaker im running all this off a avc a1-hd intergrated amp, ive done some reasearch and been told to pick a sub very wisely as it will make or break the system just wondering if you have any suggestions thanks