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