@brotw
I send the left channel to the front left sub, the right channel to the right front sub and both the left and right channels to the third sub.
In order to reproduce spatial cues at bass frequencies, don't you need dual subs sitting next to the mains?I have a pair of bookshelf speakers that I integrated with a pair of subs. I am crossing over much higher than 80hz to protect the 5 1/2" driver in the bookshelfs from over excursion during loud playback so my set up is as you described with the subs inline with the mains. I recently added a third sub and due to the extremely small room I really didn't have any placement options except right behind the listening position on the left side of the room firing into the room at a 90 degree angle to the front speakers. The third sub is crossed over at 80hz. The bass in the room became much smoother and more impactful and sounds like it is coming right at me from the front of the room. I can reach back while sitting in the listening position and touch the surround of the sub behind me and feel it moving but all the sound I am hearing is in front of me. It's really kind of wild how it all comes together.
I send the left channel to the front left sub, the right channel to the right front sub and both the left and right channels to the third sub.