If one wants guaranteed seamless integration of the subwoofer and main speakers, then it would be best to use an external active crossover. This device shall allow the user to combine both the sub and mains with optimal filters, thus restricting the precise frequencies from the mains that the subwoofer will now be reproducing. It is unwise to use mains full range, when using a capable subwoofer, as without the correct crossover, this puts the mains under more stress (higher distortion, restricted dynamics) and does not allow for optimal physical integration of the two sound sources.
I would suggest two slightly smaller subs, one placed in each side of the room, as opposed to a single larger subwoofer. This will allow better bass fill in the room and guarantee perfect integration at the optimal xover frequencies to allow the mains to perform to their potential, which is around 70-80Hz most likely.
-Chris