Thanks to all for the interesting discussion on waveguides in active monitors; it's given me much to think about and listen for.
However, I see no practical problem in rotating the waveguide in 3-way Genelecs as the MF/HF section still remains vertical. Because the LF/MF crossover is somewhere around 400 Hz (i.e. pretty low) there will be no practical difference whether vertical or horizontal, but of course the room reflection pattern will change due to different height of the woofer. The woofer directivity remains as it is and so does the MF, the only changing parameter is their relative position.
I find myself taking over the producer's role on a small-label classical production, and was thinking about these comments a couple days ago during a tracking session . . . the mains in this studio are horizontal soffit-mounted Genelec 1038s. I still feel that they're problematic in this configuration, and much of what I don't like is in this lower-midrange area that's likely related to the low/mid crossover region. It's bad enough where for the mixing sessions we'll either have to come up with a near-field arrangement that I'm happy with, or move to another facility. Either way, I was relieved that I was only making performance decisions through those 1038s . . .
Vertical mounting of course doesn't eliminate the crossover-related off-axis lobing, but it places it entirely on the vertical axis . . . and the vertical-axis listening position in the control room varies far less than the horizontal. Perhaps the use of brick-wall filters would reduce this, but I have no experience with monitors that use them.