What I mean is, I appreciate that there is a simple "cleanness" to the sound of a single driver system with no crossover and all sound coming from a single source.
Personally speaking, I’ve never heard that. I think the “magic” of a 1 single driver is only uncovered with the eyes. I’ve come around maybe 80% to think that about coaxes as well, despite a long history of favoring them.
IMO all that an unfiltered wideband driver gives you is what’s been stereotyped as “Bose sound” - no highs, no lows. Thought I would take the most basic powered Bose system over, say, a Lowther based system, any day of the week! An EQed full ranger just sounds either dull and closed in or piercing and closed in, depending on how the treble is voiced, because of the collapsing dispersion.
As for the other things in the OP:
- Open baffle / dipole concept
Open baffles excite room modes differently from monopoles, so they will sound different than box speakers: maybe better, maybe worse! They cannot pressurize a room very well, which depending on musical preferences may degrade realism.
In the midrange and treble they add much stronger reflections. Some like the effect and some find it adds a discomfiting stamp atop the imaging baked into a recording. That is a matter of program and taste. They also radiate less to their sides than box speakers.
- Horn loaded drivers / transmission line speakers
“Transmission line” is mostly marketing…. So is calling bass “horn loaded.” Either a system shifts the energy under the curve by using the back wave to add radiated energy above a given passband, and subtract radiated energy below, or it does not.
Horn midrange/treble can be the best or can be the worst! Benefits are higher efficiency and fine control over dispersion pattern. But much depends on the quality of the design as well as the suitability of the resultant dispersion pattern to one’s room configuration and sonic preferences. Also most horns throughout history have been terrible sounding!
Note that most (not all!) speakers that perform properly (flattish and smooth on axis response, smooth off axis response) use sculpted waveguides to control the tweeter’s dispersion at the bottom of its range. All horns are also waveguides.
- Electrostatic / Magnetostatic speakers
Different radiation pattern, generally narrower horiztonally, much narrower vertically.
Less efficient in key areas of the passband by up to 6dB, but more efficient down low. Closed boxes make a lot of sense for main speakers designed to be used for subwoofers, powered speakers with decent sized woofers, and powered subs. They’re harder sells as unpowered speakers, because people like bass and they have less.
You think some of these atypical speakers have a potential to match / exceed the sq of a well executed bass reflex speaker?
With the exception of closed boxes, which I find generally better in well-optimized modern systems because they’re smaller and the benefits of bass reflex are addressed with multiple subwoofers, it depends on the rigor of the design as well as program and personal taste.