OTL amplifier and electrostatic loudspeakers

Hopefully, Ralph will chime in, but in the meantime...
It looks like Quad says 100wpc/sensitivity at 86db, so your Atma's are a little on the low side of power. You should be able to drive them, but I wouldn't crank it up too high.
That said:
I would suggest buying a pair of Paul Spelt Zero Autoformers to handle any low impedance dips of the Quads-I see Stereophile measure a dip to 4 ohms at lower frequencies-not terrible, but why tax your tubes?
If you could relieve the amps from the lower frequencies with a sub/s, you'd probably be good to go.
Bob

Ralph @atmasphere on this board should be ultimate authority @gdnrbob so rightly said

I would try it with and without a set of ZEROs (www.zeroimpedqance.com).
With that speaker our customers seem to like both with and without. Overall the speaker does not have any severe impedance dips, so if you use a set of ZEROs don't use more than 2x.
The ZEROs would mostly be used to restore the highs - the amps otherwise should be fine in the bass. Make sure the speaker is at least five feet from the wall behind them.

@OP. 
Well, there you go...
B

one reason why I appreciate manufacurer presence and expertise here :-)

@tomic601 , as we both own Vandy's. I concur. A great manufacturer stands by their product. Considering that I bought my 3a sigs used, Mr. V. called me back the next day, and even the following day to see if I still had a problem. Now that is exceptional service.

Do you have Atmasphere's?
B

@gdnrbob - no , not yet.....I have my Dad’s old but hot rod MC240 in my “vintage” system  where the 30 wpc or 60 wpc OTL would shine... and then the Treo in a Condo where monoblocks sans tube cages would be in danger of the 4 and 5 year old nephews.... and the Vandersteen M7 amps in my reference system..... so I am for now an ardent admirer and wannabe kit amp builder...
the war department and my sanity stand as temporary obstacles....

graphitteller
  OTL amplifier and electrostatic loudspeakers
"Atma-Sphere M50" a loudspeaker Quad 988

With this sort of power from the OTL’s and an output impedance of 4.1Ω, on a speaker that’s only 83db!!, I would say no. Get the right amp, and not a expensive interim fix with autoformers.

M50-60
60 watts per channel into 8 Ω load
45 watts per channel into 4 Ω load
80 watts per channel into 16 Ω load

https://www.stereophile.com/images/archivesart/quad989fig1.jpg

Cheers George

We have enough customer running Quads that I can safely say that they are a great combination.

We originally designed the amps to be used optionally with an outboard autoformer. Thus years ago we marketed the autoformer as the Z-Music autoformer.

The design of our amps is based on the idea that feedback adds distortion that is easily heard by the human ear as brightness and additional harshness.
So we designed the amps to have good linearity without feedback. Triode power tubes, single stage of gain, class A2 operation, fully differential from input to output and direct-coupled output.
It was obvious that such an amplifier would have a higher output impedance (we of course built amps with feedback, some of them switchable, so it was easy to hear the detrimental effects it had). So the Z-Music was designed to allow the amps to deal with lower impedances. It was pretty effective; in his review of the MA-1 in The Absolute Sound Steven Stone used the MA-1 with a set of Apogee Full Range loudspeakers, which are 1 ohm impedance- and it worked very nicely! Other than impedance, that speaker was easy to drive.

We don't make the Z-Music anymore as the ZEROs had slightly better performance. Neither product was/is expensive, so this has proven over the decades to be an elegant solution for lower impedance speakers coupled to lower powered OTLs and other amps like SETs that normally have a higher output impedance due to no feedback.

We occasionally get the question- doesn't that eliminate the best qualities of the OTL? And the answer is no because an output transformer like the ZERO or Z-Music isn't possible unless the rest of the amplifier already has a fairly low output impedance and no DC at its output (both devices don't block DC as they only have one winding, and the turns ratio is extremely low so no distributed capacitance to kill the bandwidth). As a result, both products have bandwidth that is well in excess of the amplifier used with them- the ZEROs go for 2Hz to 2MHz! Usually the output transformer is the bandwidth limit of the amp but with this system that isn't the case. In the meantime the amp still has all the qualities that make it work as I described above.