Does the EP600 in the horizontal configuration suffer at all in terms of SPL/clarity/etc. in comparison to the vertical configuration?

M, it wouldn't appear that it would make a significant difference. Those frequencies of course are essentially omnidirectional, so there wouldn't be any dispersion factor involved. As far as boundary reinforcement goes, the horizontal placement would put the cone center about a foot closer to the floor, but also about a foot farther from the side wall(assuming a corner placement was used). The resulting response should be essentially identical.