OTL Tube Amps and Full Range Electrostatic Speakers

"...About $6,000/pr,"

Actually monoblocs and more like $6500 each. 

Thanks to roberjerman, bdp24 and kalali for their comments.  I have heard the Futtermans on Quad 57's and it was a great combination. I have used a number of highly modified ARC tube amps on Quad 63's, 989's, and 2805's with good results.  I have had a number of Sound Labs - most recently the A3's - as well as many Martin Logan hybrids the CLS IIZ and most recently the CLX's.  While it is probably not worth delineating all the amps - both solid state and tube, Pass, Rowland, Levinson, Krell, ARC, CJ - that I have used over the years with these speakers - I have not used OTL's with the ML or Sound Lab speakers.  There have been a number of OTL manufacturers over the years, but currently I am aware of only Ralph Karsten at Atma-Sphrere and Roger Modjeski at Music Reference who are currently producing OTL amps.  The CLX is perhaps the most fussy speaker that I have had both in setup, room requirement for optimal performance  and the upstream equipment needed to optimize its performance.  I suspect that is why one sees CLX's on the used market.  Has anyone had personal experience with OTL's on either the ML CLX's or the larger full range Sound Labs? 

Some more OTLs that have been popular recently... (I haven't heard them)

https://www.lineartubeaudio.com/new-page-1/

We have a number of customers using the RM-200 with their ML speakers. It's an excellent amp for the job. OTLs can also sound great, if Atma-Sphere I would recommend the MA-1 which is 100 watts. I'd recommend putting the autoformer in between the amp and speaker, but it may not be necessary. Ralph could advise as well.

There is some debate on whether the Berning patent for his design and that is now used in the LTA amps is actually an OTL circuit. IIRC there is a high frequency transformer that follows the tubes in the circuit. Therefore, some feel that technically it can't be an OTL and I believe Berning more accurately refers to the design as Zero hysteresis since that high frequency transformer does not behave like a traditional output transformer. So the other side of the argument is if it doesn't behave as an output transformer it is still an OTL (output transformer less).

Power increases into lower impedances!

@roberjerman

One thing about ESLs is their impedance curve is not also a map of its efficiency, like it is for most box speakers.
Instead, its impedance curve is a result of being largely a capacitor. For this reason, what is desired is the ability to put out constant power over the range of impedances presented by the speaker rather than constant voltage (which results in increased power into lower impedances per the quote above). Of course, no amplifier really does make constant power but they can come pretty close, especially if assisted by a set of ZEROs (www.zeroimpedance.com) since any tube amp that is designed to make constant power (like ours are) will tend to have a higher output impedance.
For more on this topic, seehttp://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php