Recommended Speaker List for Atma-Sphere OTL Amplifiers.

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?

Actually we used an Audio Note 300B, an Audio Note Kit 300B parallel single ended, a Mastersound 32B (crushed the other SETs). VAC Phi 200 and Atmasphere M-60s. Preamps used were Essence Jasper, CAT, Audio Note, Thor Audio and an older VTL. The preamps universally sang the praises of the M-60s. To understand the inherent limitations of SETs you need to compare them with an OTL. You will instantly hear the limitations of the transformers, the inherent problems with the lower power and the quick clipping which sounds like the amp is producing power but it is really a bunch of gunk. The push-pull VAC did very well , but lacked the texture of the Mastersound and was completely outclassed in every way by the Atmasphere. Most of the listening was done on a pair of DeVore 0/96s which is a very easy OTL friendly load. 

Thank you, I asked because I can't comment on the 300B SET as I haven't had a chance to compare one on my system, I have a PSET in which I tried many different tubes combinations (not 300B) and when it starts clipping and distorting is not very pleasant, still better than a SS but obviously distorted. The Atmas on the other hand (and this is not apples to apples as I have MA-1s) are wonderful driving 10 ohm speakers.
If you are listening to low volume the PSET is pretty good.