Recommended Speaker List for Atma-Sphere OTL Amplifiers.

Dragon, it all depends on how and what you like to listen to. If you are a rocker and like to play loud than an efficient horn system like those mentioned above would be the way to go. But if you tend to listen to more acoustic instruments, jazz and classical at moderate levels then the mid range purity of a full range ESL would be best. Something like the Sound Labs 545. I am not crazy about ESL-57s because they are just too fragile and in all reality the Sound Labs is a much better all round speaker. It is much better in the mid bass and it's high frequency dispersion is much better as well as being tough as nails. Ralph also insists they match up with his amplifiers beautifully. The 545s are also a lot smaller than the big boys. I believe they are narrower and taller than the ESL-57s and would take up less floor space. They are less sensitive than a horn system and will not go as loud but still 8 watts will get you to 95 dB, a comfortably loud volume and theoretically you could hit 102 db with a 60 Watt amp, louder than most of us listen.

I’m a little late to this thread sorry.

 I’ve come across some older M60 mk2 amps a friends has at his audio shop the price is right so I’m seriously looking at them. Question is will they drive high efficiency horns like the current klipsch heritage line. I’ve currently been trying  some modded heresy 3s with the thought to move to the new Heresy 4, Cornwall 4. Or La Scala in the near future. 
I’m very new too OTL Amps but am quite versed in SET etc. I’m coming from SET 300b Amp. So tubes are not new just OTL. 
Also I read the amps can be run with less tubes at lower power is this correct? 
I am considering a trip to the shop for and update-recap-etc as required.  Probably new tubes as well as the ones in it are unknowns and miss matched. I suspect the previous owner pulled his good tubes when he traded it. 
Thanks I’m advance 

glen 

I believe you should have no problems running the M60s with the Klipsch line, if you are coming from the SET world where typically you manage less power than what this OTL can offer you should be fine. Yes you could run the amp removing tubes but I personally wouldn't, others have said and I have my subjective feeling and like many other cases that running with full capabilities will provide better sound at lower volumes with high efficient speakers than with same settings and tubes removed. I have MA-1s BTW and I run all 28 tubes all the time with high efficient speakers, the power tubes in the Atmas are very inexpensive and they last a long time because the way their circuit operates, the mk3 revision improves this as well if I'm not mistaken so it might be worth sending the amps to Ralph if you have the financial means of course to check them and reactivate the warranty and upgrade.

Which preamp are you planning on using?
How big is your room and how loud you listen to music?

My concern with Klipsch speakers would be possible dips in the impedance curves of each speaker. Efficiency is only part of the issue.

When we compared the M-60s to several SETs the results were significant. The M-60s were like water while the SETs like chocolate milk. The other thing that we found is that even the 20 watt parallel single ended amp ran out of steam very quickly and started to distort in a very obvious, albeit pleasant way. 

I personally was thinking mainly on impedance more than in sensitivity terms, you are correct impedance is big part of it, most of klipsch is usually 8 ohm nominal and above 93 sensitivity, I have P37f as secondary speakers and very easy to drive with the Atmas, mains are AudioKinesis which are 95 dB and 10 ohms.Anyway, could you recommend me an amp like bourbon? Not much into chocolate milk.
Joking aside, the preamp used could have a lot to do with your SET comparison but I have to ask which type 300b?