If its not common knowledge than it should be: all planar speakers need to be a minimum of 5 feet from the rear wall!
Any closer results in tonal colorations, the most pronounced being in the bass. Oddly, the only speaker I have seen that worked at all well at distances closer was indeed the '57, but invariably the owner was using a transistor amplifier, which cannot play bass on the speaker. Nevermind- the backwave reinforcement 'sort of' takes care of that. But put a good tube amp on a setup like that and the owner complains of too much one-note bass.
Any closer results in tonal colorations, the most pronounced being in the bass. Oddly, the only speaker I have seen that worked at all well at distances closer was indeed the '57, but invariably the owner was using a transistor amplifier, which cannot play bass on the speaker. Nevermind- the backwave reinforcement 'sort of' takes care of that. But put a good tube amp on a setup like that and the owner complains of too much one-note bass.