The disappearance of the traditional amplifier

the low-mid transition is that of a horizontally-placed design.

Yes but at 410 Hz (the crossover) the wavelength is about 2.7 feet (30 inches)! Given the drivers are only about 12 inches apart the lobing should not be apparent except at extreme angles (well above 60 degrees).

I wonder if you are hearing the effect of soffit mount - this has a significant impact on lower mids and mid bass compared to free standing (you no longer have comb filtering from reflections off the wall behind the speaker so that bass becomes solid, more tangible, more directional and less diffuse)

Yeah, your logic makes complete sense . . . and there are of course innumerable differences in the environments and installations - including those where I've been able to compare vertical vs. horizontal installations of this type.

But 400Hz does lie in a region below where most types of room treatment are effective, and yet above the region where a "control-room-sized" (whatever that is) room is exhibiting primarily modal behavior. In larger spaces (medium-to-large-venue sound reinforcement) it's pretty much a given that well-controlled directivity in the 400Hz region is very important, but in smaller rooms . . . there are many opinions.

For some reason I also have this association with other horizontally-configured monitors, namely Westlakes. But it seems like all the people I've known who have Westlakes also monitor at ridiculously high levels, and maybe that's why I have a bias against monitors that look anything like them . . .

But 400Hz does lie in a region below where most types of room treatment are effective, and yet above the region where a "control-room-sized" (whatever that is) room is exhibiting primarily modal behavior.

Agreed - the frequencies between 60 and about 500 Hz are the principle reasons to soffit mount speakers. You can avoid the inevitable suckouts that occur with certain lower mid and mid bass frequencies. The suckouts are often not that huge (6 to 9 db) but they are harmonically related and are therefore usually quite audible. If you heard a certain unusual mid bass thickness in the vocals then it might be the affect of the soffit mounted Genelec's versus what you are use to hearing all the time from free standing near field speakers.

Interesting thread indeed - I enjoy myself a pair of GENELEC 1034A for some years and did not find anything globaly better. Would be happy to get pros or cons opinions.
Stephan / Miami