In need of some advice on a matching a sub

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.

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.