Paul McGowan gets asked about rotary subs.

Would love to hear with my Acoustat Model X’s.  Over the years, I’ve tried probably ten subwoofers with them and have never found one yet that’s fast enough.

@bdp24, seems like we share a few perceptions.

For years I've set a goal for 40 Hz bass in speakers I've owned.  Not necessarily flat down to that frequency, but at least reasonable output.  I'm a big jazz fan and the 42 Hz (as I've typically read) of the open E string on a bass defines the minimum of what I expect to hear.

Basic need for lower bass.  Not that many instruments extend below 40 Hz, nor is there an abundance of music scored in that range.  But I too learned that the sense of space in large recording venues is better captured by systems which extend into the lowest octave.  So that elusive sense of realism of live experience is better supported by deep bass capabilities, even when not attempting 16 Hz organ notes.

I don't remember the year, but clearly recall the experience of Bill Johnson coming to a San Diego dealer to set up an ARC tube system with a pair of Tymphany speakers.  Still, one of the better audition experiences I've had after decades in this hobby.

Unfortunately, at my age I'm looking to simplify my system.  So while the larger rec room where I had hoped to set up a good second system (where ET 8Bs would have been contenders) was a dream, I must admit it is not likely to happen now.

@curiousjim: Allow me to suggest a subwoofer that is ideal for your Acoustats: The Rythmik Audio/GR Research OB/Dipole Subwoofer.

Planar lovers (especially electrostatics) have been looking for a sub which can "keep up" with their loudspeakers since QUAD and KLH introduced their ESL’s in the 1950’s. Everything form the 24" Hartley woofer in a huge enclosure to a transmissionline-loaded KEF B139 woofer (David Wilson used two B139’s in each of his original 1970’s WAMM model, for midbass frequencies.). The Finnish company Gradient was for a time making OB/Dipole subs for use with both the QUAD ESL and QUAD 63. The design was good, it’s execution not-so-hot. It was reviewed by Robert E. Greene in TAS, if I remember correctly.

Traditional engineers will tell you (the guys at SVS did me) that a woofer can’t be "slow", as it is reproducing only "slow" frequencies. That is of course an over-simplification of a more complex situation. In his excellent series of Tech Talk videos on YouTuve, Danny Ritchie of GR Research discusses all things loudspeakers, including subs. I won’t bore everyone with information better explained by Danny, but suffice it to say that the reason subs have never sounded "fast enough" to blend seamlessly with ESL’s can be explained. Which Danny does.

One important factor is that monopole subs (whether sealed or ported) don’t propagate sound in the same manner as do dipole loudspeakers, nor "load" the room the same way. Employing either dipole planar woofers (as Magnepan does) or traditional woofers used in dipole fashion (a woofer mounted both front and rear in an enclosure, in polarity opposite each other, just as the front and rear of a dipole loudspeaker) solves that problem.

Magnepan has been working on a dipole sub (but it is not open baffle) for a few years now, and has demoed it at a few high end dealers (and hi-fi shows, I believe) around the U.S. The woofer system is the basis for an upcoming Magnepan model: The 30.7 For Condos. It will have Magnepan planar-magnetic drivers for midrange and treble, the woofers of course for bass. Magenpan will also offer the dipole sub as a separate product, for use with their other models (or the loudspeakers of other companies). Eminent Technology already offers a similar woofer system, but it is available only in their LFT-8c loudspeaker. Perhaps ET will sell the woofer system separately? It is offered as an upgrade for current LFT-8b owners, $1500/pr, I believe. Complete with power amp and DSP for crossover functions, time alignment, and equalization.

But the best answer is the amazing OB/Dipole Sub from Rythmik Audio/GR Research. Either two or three (your choice) 12" servo-feedback "free-air" woofers mounted in an open baffle frame, low frequencies (up to 300Hz) propagated both front and rear, just like dipole loudspeakers. The best thing I’ve found for use with my QUAD ESL’s, Eminent Technology LFT’s (both -4 and -8b), and Magneplanar Tympani T-IVa midrange/tweeter panels (I have no room large enough for the Tympani’s bass panels. They are HUGE!).

The sub is available only as a kit (which contains the plate amp---which provides the best controls in the business---and the woofers.). The OB/dipole frames are your responsibility, but GR Research has a company in Canada which makes the required OB frame, shipped as a flat pack. I know that sounds like a lot of work, but the frame is very well designed and fabricated (on a CNC machine, the MDF very high quality and thick), and easily assembled (only wood glue and clamps needed). Reasonably priced, too.

The genius speaker/crossover designer Siegfried Linkwitz came to the conclusion that open baffle/dipole loudspeakers are the way to go, and designed and sold (he died back in 2018, but arranged to have a colleague continue to market his designs) a loudspeaker that includes a sub of the same design as that of GR Research. But the latter is the superior sub, imo.

End of sales pitch/shill. ;-)

@bdp24,

Thanks for the tip. When I get a chance, I’ll give Danny a ring. Years ago, I built my second to last sub with two Dayton 8” drivers. I got very close,  but I could never get all the way. 

I have read many posts in which @bdp24 praises the ET LFT8b's and having owned (and still own but rarely use) Magnepans, I finally decided to look up an ET dealer close to where I live.  I called to ask if I could visit and listen to a pair but was disappointed to find out that he no longer carries the speaker since there was never much interest in it and he sold very few.  I really would like to hear these speakers and have identified other dealers within a few hours drive.