Thank you for the recommendation, @ohlala .
I really like Ralph’s amps and pre-amps (I have an UltraViolet too), so just about all of my speaker designs have his amps in mind. Specifically I usually shoot for compatibility with the S-30, on the theory that if a speaker works well with the S-30 and with solid state, it’ll work well with everything in between (which would include the M-60). Which is another way of saying that I pay a lot of attention to the impedance curve, and make it possible to adjust low-end tuning to take amplifier output impedance into account.
Given that your room is open on two sides, imo we’d want a radiation pattern that has relatively little room interaction, at least from early sidewall reflections. Assuming one of those openings is on a side wall, you’ll have zero reflections from that side but a normal amount of early reflections from the other side wall, and the imbalance can skew the imaging. But if the speakers have a fairly narrow radiation pattern to begin with, along with strong toe-in, we can minimize the early sidewall reflections off of that one wall and therefore reduce the discrepancy.
Two of the walls having big openings implies that we may need a lot of bass energy, as those two big openings may act like very effective bass traps. Or they may not. Anyway imo a fair amount of adjustability in the bottom couple of octaves is probably desirable.
Finally those big openings mean that the music will flow into the adjoining rooms, and it might be nice if it sounded good in those adjoining rooms as well. This implies that we want the radiation pattern to be fairly uniform so that the reverberant field has a natural-sounding tonal balance. This is something a good horn system does well, resulting in good sound "in the next room", where we have no line-of-sight to the speakers so all we can hear is reverberant sound.
Ohlala mentioned that my designs can have some of the characteristics of dipoles. I was a SoundLab dealer for many years. I like the natural timbre and deep, enveloping soundstage you get from good dipoles positioned far enough out into the room, but such positioning isn't always practical. So I have a technique for achieving that sort of presentation which does not require the speakers be pulled way out into the room.
Obviously my biases are showing through in this post, so take it with as many grains of salt as needed, and best of luck to you in your quest.
Duke
dealer/manufacturer