:) mmmm..interesting! I'm getting the picture with the time relationship and the rear wall. Can the face of the speaker be pushed toward the edge, with the rear of the cabinet going beyond the concentric circle, but the front still within? These are not rear ported.

--Rick

I dont have any of their speakers but I found this to be interesting reading.
I hope it gives the info you are looking for. It is where that picture came from. Their is something about a "golden ratio" on there too.

aanvilaudio.u-net.com/

If your talking something with like a 10" speaker or larger than I would say at least 6-feet and within an average range of 6-10 feet depending on speaker and cabinet size. They may not work very at all at any spread/distance if used in a small room with a low ceiling.