Probably worse since it was designed differently for better horizontal dispersion when oriented horizontally . Though I suspect a vertical VP180 would sound better than a horizontal M80.
I'd still like to see someone try using VP180s vertically as mains. Maybe if I ever get duals for centers. Hey, maybe JC can try. While it's true that it was designed to be a horizontal center, I still wonder if the driver arrangement could sound decent vertically, since, Dean, don't you prefer your M80 tweeters elevated above ear level? If you made the mids of the VP180 at ear level, the tweeters would be above and below your ears instead of at ear level. You'd need a stand, which would be odd for such a big speaker, but I just wonder if the current M80 design (and that of a lot of other loudspeakers) is a compromise since most companies don't want to ship speakers that go from your floor to your ceiling, so it's just easier to make a speaker with tweeters at the top of the enclosure.
