Five feet from the front wall

I should add,

Listening distance- distance from centerline of the tweeter to the listener’s ear.

Well with my Clarisys Minutes (Planars, Ribbons), I have found 5 feet from the front wall with some diffusors behind them to be ideal. No absorbers nearby. I do however have acoustical tile over the drywall ceiling and carpeting on the floor.

The distance between them also matters. Too far apart and you lose the center image and some treble. In my 17W X 25L x 8H foot room I have found about 8.5 feet from the center panels to be about right.

ozzy

On the "front wall" being in front of the listener vs behind the speakers:

People (listeners) are aware of their position in time and space and can contemplate their position relative to objects in their environment.  Speakers are not aware of anything, so they should be disqualified from having an opinion about what's in front, back, to the side, below, on top, or inside of them.  They just do what they are told to do via electrical impulses controlled by the listener.  So, I vote for the listener, and defining walls relative to the listener.

@mijostyn 

"Because loss of volume with distance is much less severe with line sources"

My understanding this subject is something like this:  sound attenuates 6db as distance doubles.  With a line source (array) the height of the array pushes the phenomenon back relative to the height of the array.  So, if the array is 8', the attenuation process is pushed out 8' before it begins the process of attenuation.  Correct?

@waytoomuchstuff 

No. With a point source the volume drops at the cube of the distance. With a line source it drops at the square of the distance. Volume drop off is an order of magnitude less. As for height that depends on the frequency. For a driver to act as a line source it has to be taller than the longest wavelength it is to reproduce with one very cool exception. Low C has a wavelength of 56 feet. A little hard to get a 56 foot tall speaker into most houses. BUT, if the ends of the line source butte up against barriers like the floor and the ceiling, the line source becomes infinitely long as if the floor and ceiling were mirrors.  This is the rational behind 8 and 9 foot tall speakers. In the case of ESLs it turns a modest gentleman into Axel Rose. Add a line source subwoofer array and bring on the concert! Line Source systems have a very different presentation. Everything is larger, front row vs rear of the hall. The image is more stable and the overall sound more comfortable as in less prone to sibilance and distortion at high volumes. Many people prefer Point source systems but usually not because of the sound, more likely the size of the speakers. I had a new friend over who is a Bricasti dealer. He listens to Franco Serblin speakers, very fine Italian point source speaker. His first comment was, "I'm not use to this! There is so much bass!" The system runs flat down to 100 hz then rises at 3 dB/oct down to 20 Hz. The treble falls off from 2000 Hz at 1 dB/oct. This is very standard target curve for most rooms. Dirac live usually starts with this curve. Most systems do not project much under 60 Hz. The frequency response specs are taken at one meter. 

My speakers are 5ft off the ceiling looking at them while sitting on the floor with my back to the front wall.  They sound fine that way.