Interesting, when I had Quad ESL-63s I came upon almost the complete set of the early mono Mercury Living Presence LPs. They sounded pretty good, and the discs were in pristine condition. I eventually played them using only one Quad. They sounded so much more right that way I always listened to them with one speaker only.
Monophonic recordings (IMO) sound much better played through one loudspeaker. Unfortunately, some digital oriented preamps (such as the Benchmark HGC) don't include a balance control. If you use a PC for a head then channel selection can be done easily in software. For analog sources it's not quite as easy--one way is to unplug one of the channel cable leads from the phono preamp or tape recorder. Not a very elegant solution.
For most people it's not an issue since they are not often listening to monophonic recordings. Or if they do, they just adapt to the two loudspeaker blend and don't worry about it. Even so, with analog sources it is often not possible to maintain a consistent phantom monophonic 'center image'. Typically a balance control is required. This is because analog sources (especially phono cartridges/records, but also magnetic tape) often have mismatched channel levels.
The original Quad was designed during the mono era. It suffered from the usual electrostatic horizontal dispersion problems, making the best listening spot fairly limited. Contrast it with corner horns, which spread the sound into the room, allowing for a less constricted monophonic listening area. The Quad and the Klipsch successfully survived into the stereo era--however because of the vertical dispersion limitations, Quads were typically spaced to create a triangular 'sweet spot', with the listener at the apex.
The Klipsch required two corners, often widely spaced (because of architectural constraints)--a geometry which could ruin any semblance of a center 'phantom' image. A work around was the placement of a third center speaker between the two corner horns, summing the left and right channel in one or another way. Paul had a circuit devised for that--I never heard how that sounded. It was a kludge, but what else could be done?
Getting back to the original Magneplanar, I never was able to figure out exactly how one was supposed to place it in the living area, what with its three panel 'room divider' form factor. Was it supposed to remain folded in a zig zag? Folded out in an arc?
