X-over for adding a sub to speakers

[Unobtainium]

Following up on that great read about sound engineers and recording,etc, I was wondering if there are any high quality x-overs to mate a sub to your speakers, not LFE. I used to just take pre-outs and feed subs the signal of the front left and right channels. The end result was good, but a proper x-over would be great.
This is what I'm thinking:
Front left- studio 100 crossed with a sub-12
Front right-same
Center-same
Rears-studio 100's full range until the coding includes more bass requiring subs.
LFE-2-FW-18's

Richard Hardesty used to talk about a similar set-up being optimal.
He used Vandersteens with his speakers and an Energy 18" for LFE.
He had 5 subs(he thought a powered center too much for dialog)
Please discuss the merits of this, and any equipment to help pull it off.