In need of some advice on a matching a sub

Phew, that integrated amp has one long manual.
In any case I see it has subwoofer features so you can either run them Large or small-meaning it can do crossover adjustments.
That being the case, you shouldn't have any problems integrating them with your Kef's. Just about any quality sub should have no problem.
JL, REL, HSU, etc.

Wasn't worried about the crossovers more the tone of bass as the Kefs already have 6 subwoofers 4 10s in the front and 2 8s in the rears I don't want a tonal difference from the sub to the speakers

Well, the crossovers should cut off frequencies so that the sub and speakers aren't generating the same signal at the same volume. This should eliminate the tonal differences you are wary of.
It's sort of like your speakers which have crossovers to allow each driver to work at its' optimal range. 
If you go with a high pass filter, then your Kef's would reproduce frequencies above the cutoff, while the subs provide all frequencies below. It also allows your amp to run more efficiently as lower frequency require more power.

I still want the bass from the speakers tho, they have subwoofers in them and they work well at the moment it's just for that extra oomf when an explosion hits really 

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.