Why not more popular?

It is hard to make a speaker that is wanted and cherished by all audiophiles there are just far too many options for that to happen at this time of audio history.

@jaytor, there are a few factors which pretty much answer both the major and minor questions in your post. Because I find this subject so interesting, I shall endeavor to remember to get to them all while also getting "into the weeds", as they say. Geez, I hope that doesn't sound as pretentious to ya'll as it does to me ;-) .

But first: I can't believe this needs to be said, but it apparently does. Martin Logan has not for years been making what we call open baffle loudspeakers. Not for years, nor in fact at all. Martin Logan's electrostatic loudspeakers are not open baffle loudspeakers, they are dipole planars. So are all the other "full range" ESL's (as opposed to ESL tweeters, like the RTR's in my pair of ESS TranStatics) that are being made, or ever have been. So are the magnetic-planar loudspeakers of Magnepan and Eminent Technology, and all the full range ribbon loudspeakers, past and present.

An open baffle loudspeaker is specifically defined as a (usually) dynamic driver (or drivers) mounted on a baffle, pure and simple. No sealed or ported enclosure, the driver(s) open both front and back to the enclosed room in which they reside. To be sure, some OB designs have included a dynamic woofer and either a magnetic-planar tweeter (as in some GR Research models), or a ribbon one. The point is, the drivers are not loaded by an enclosure (with technical implications. See below.)

OB's have a long history (my first loudspeakers were OB, purchased in 1968), but have remained largely unknown to most audiophiles due to a very simple and important reason: they have been almost exclusively a DIY product, not a plug-and-play one. There is a very active OB loudspeaker "underground" community, one whose members include guys like Nelson Pass. OB enthusiasts are in one way just like the original 1950's hi-fi amplifier designers, who started making their amps on the kitchen table.

Because OB's were built, not bought, there were never any OB loudspeaker dealers, no magazine reviews, no advertising, no nuthin'. And because there was no market for them, they have for years remained largely a DIY underground phenomenon. How many guys do you know who would even consider building a loudspeaker? Kenjit would, but he can't decide which of his designs is most perfect ;-) .

The word about OB's took a giant leap forward when the design genius known as Siegfried Linkwitz (a neighbor and close friend of Nelson Pass) started publishing his papers on OB design, and introduced a number of OB loudspeaker kits. The ultimate realization of his OB design was the model mentioned by another poster above---the LX521, and is very highly regarded amongst OB enthusiasts.

Danny Richie was another long-time OB enthusiast and designer, and in the 1990's started a company (GR Research) catering to the DIY loudspeaker community. His loudspeaker products were all kits, but not all OB designs. He offered sealed designs as well, of both loudspeakers and subwoofers. While admiring Linkwitzes OB designs in most regards, it was not without reservation. Danny is that perfect combination of knowledgeable designer and critical listener (as was Roger Modjeski), and in his products address every failing he heard in the work of Linkwitz.

For instance: Danny is a fanatic about resonance, and he heard a problem in the OB sub section of the Linkwitz 521. He found the 3/4" side panels would flex when confronted with deep bass, producing a sound he describes as "buzzing". His solution? Double the sub side panels to 1.5" and line them with Norez, a resonance damper he designed and has manufactured for him. He also greatly disapproves of the digital x/o inherent in the 521, but that's a subject for another time.

Danny hears resonances in just about all commercial designs, and for his sealed sub designed a "double-box" enclosure design: an inner box and an outer one, with a layer of sand between them. Whatta nut! Danny will tell you what's "wrong" with most OB's (though he expresses his approval of the designs of Clayton at Spatial. They are friends.), and why he does things his way. For instance: he finds that an OB with a large baffle (needed to lower the frequency as which the front and rear waves meet and cancel, commonly known as dipole cancellation. That frequency is distance related: the greater the distance between the front and rear of a driver, the lower the frequency at which cancellation occurs) draws attention to the baffle, preventing the creation of a deep layered sound field. So, to create the distance required between the front and rear waves, in place of a large, flat baffle, Danny uses a baffle only just large enough for the driver, and creates the needed front-to-back distance with side "'wings". To see it in pictures, watch any number of his GR Research YouTube videos.

But OB design is much more that just the baffle. A driver used in an OB application needs to posses different technical specifications and performance characteristics than does a driver used in a sealed or ported design. Danny designs drivers for both applications, and has them manufactured for him in India. He offers his 12" woofer in both OB and sealed/ported versions, each optimized for their application. His 12" woofer is identical in most aspects to the 12" woofer Rythmik uses in their F12 sub, but with a paper rather than aluminum cone. Danny prefers the lower mass and timbral tone characteristics of a paper cone, Rythmik's Brian Ding prefers the aluminum's great stiffness.

It's been a coupla years since I spoke on behalf of the remarkable OB/Dipole Servo-Feedback Sub co-created by Danny and Brian, so I hope no one minds if I do it again now ;-) . I am not alone in considering the bass produced by the Magneplanar Tympani bass panels my standard in bass reproduction, even if their maximum output is limited (Harry Pearson agreed with me). The GR Research/Rythmik OB Sub is the only one since that could compete with the Tympani. There is one fanatic who has an Eminent technology TRW-17 Rotary Woofer for deep bass (40Hz and below, flat to 1Hz at 120dB), a pair of Tympani's for bass, and a pair of Martin Logan ESL mains. Yeah, baby!

Anyway, Danny was already offering an OB sub, and when he discovered Brian Ding's new Rythmik servo-feedback woofer had an epiphany; the mating of OB and servo-feedback, to create a new standard in bass reproduction. Both men are located in Texas, and ended up putting their heads together in the project. Brian Ding says he finds the OB Sub to be too "lean", missing the weight and heft of sealed and ported subs. Danny disagrees! All I know is that the OB/Dipole Sub reproduces the sinewy timbre of an upright bass, the fat punch of my 24" Gretsch bass drum, and the lower registers of a grand piano better than anything other that a Tympani.

GR Research provides plans for both an H-frame OB structure into which the dual or triple woofers are installed, and an W-frame design (two woofers only). Danny has a woodworker making the H-frame as a flat pack kit, very simple to assemble: just wood glue and a coupla clamps required. Fantastic frame, side panels and top and bottom 1.5 inches thick, baffle 1" thick. Makes the Linkwitz look like a 90lb. weakling!

The Rythmik plate amp included in the Sub kit includes a 6dB/octave dipole cancellation compensation shelving network, which counteracts the roll off endemic to dipoles. Not just great bass, but plenty of it. It is the only sub I consider adequate to mate with planar loudspeakers. I use them with both ET LFT-8b and LFT-4 magnetic-planars, and QUAD ESL's.

What I haven't heard are the OB loudspeakers GR Research offers. Jaytor, what beer shall I bring? ;-) 

Oh, and by the way:

The Magnepan "Concept" loudspeaker---the 30.7 For Condos---is basically not that different from Danny’s NEO Line Source/OB Sub design.

The Magnepan Concept has a smaller version of the 30.7’s midrange/tweeter panel, with an OB/Dipole woofer assembly in place of the 30.7’s large woofer/bass panel. That OB woofer has multiple small-diameter woofers (6.5", I believe)---six per side, iirc. And dipole-cancellation compensation, not unlike the GR Research/Rythmik.

Reportedly to sell for less than the 30.7, if it is ever put into production. For now, a pair of the GRR/Rythmik OB Subs, with a planar of your choice (ESL, magnetic-planar, ribbon), or an OB loudspeaker (one of the GR Research kits if you’re adventurous and confident, a Spatial if you’re not) will get you very close.

If size matters, the ET LFT-8b is only 13" wide and 5’ tall, much smaller than the Magnepan 3.7i. And it’s LFT magnetic-planar drivers are push-pull designs, with magnets on both sides of the Mylar diaphram for ESL-level distortion, unlike the single-ended 3.7i midrange driver. But the 3.7i does have a nice ribbon tweeter. The MG1.7i, only $300 cheaper than the LFT-8b, suffers pretty badly in comparison to the LFT-8b. I did that comparison.

Some of the comments are regarding open baffle seem, honestly, uninformed. To compare yesterday’s OB without listening to today’s OB and saying it’s a fad is pretty funny or sad. Box speaker’s have several limitations by design which OB resolves quite nicely. Yes, they look different, but if you have an open mind, truly listen, you will be well rewarded.  I did and created this review https://youtu.be/R9VeZOdatao