Imaging and soundstage

What if you have two woofers in each tower? Would you then average the distance from the center of the two woofers?

Cosmic_void, I believe the Allison rule is a good general-type rule. The strategy of this rule apparently is to have the distances as different as possible.

Therefore, averaging between the two woofer would probably work as well as any other alternative. Although, I'd probably try applying the rule first with the lowest woofer.

I can only say that my initial speaker placement worked quite well and then I stumbled across the Allison rule in an old issue of TAS. My measurements turned out to be near exact. I've since mentioned this rule to one other person and now he claims he's not able to go to sleep at any decent hour as this application has transformed his entire musical presentation into another league. And he has Maggie 3.6 speakers.

-IMO

The way *I* understand it:-

Yes, seating distance from wall behind the listener does play a role in imaging & soundstaging.
The room will, usually, develop resonances between the centers of the wall behind the speakers & the wall behind the listener - i.e. along the center long dimension of the room. If left uncontrolled this will *likely* ruin your soundstaging & imaging. This could be fixed with some absorption directly behind the listener such as tube traps (what I got) or a low (3')LP shelf or book case. I found that it helps the most in the "warmth" region, which I understand to be the 100Hz - 300Hz region.
From my personal experience I found sitting 4' or more away from the back wall took out the back wall effects for me. My calc. show that 4' is 3.53mS if sound travels 1130ft/sec. So, it *appears* that the sound hitting the back wall, approx. seated ear level & higher, is not direct sound from the speakers & so it is weak signal. So, if one sits 4' away then the reflected signals off the back wall get attenuated enough by the time they reach your ear that they have very little effect.
I've seen some people use diffusion on the listener back wall off to the sides of the listener so as to improve hall/studio ambience. I personally haven't tried that as yet.
IMHO. FWIW. YMMV.

Read this ... it describes how the brain processes reflected sounds, and how the imaging of speakers depends on the delay of the reflected signal path (relative to the direct path) being outside a range.
http://www.audiophysic.de/info/aufstellung/index_e.html

With this in mind it should be OK to have the rear wall immediately behind you (i.e. right next to your head), or several feet behind you.

My personal experience is that several feet behind is better, not because of time delays and the like, but just because sitting close to the rear wall reinforces the bass too much, making the music sound congested.

My minimal experience echoes (Ha!) Rives suggestion. Interesting, with out any previous knowledge of Stehno's recommendation, I found my speakers to be with in 3" of his guidelines at the longest length and dead on with regard to the two others.