I have been an Atma-Sphere customer for some time. I use the M60s in my system. They drive either Spendor 1/2e or Quad ESL 57 speakers. For speakers with difficult loads I use a set of Zero autoformers between the M60s and the speakers so that the amps sees a higher impedance. As Ralph has stated many times on these forums all amps behave better driving higher speaker loads.
Also, I would note that an Atma-Sphere OTL will put out less power into lower speaker impedance loads and more power into higher loads vs. a high current sold state amp that doubles power as the impedance is halved, but has reduced power as the load doubles. Ultimately I pay less attention to power these days. I'm more interested in how much distortion the amp is going to put out (it will put out less into higher loads) and if I can get my desired SPL level in my room with the amp and speaker of my choice. I find 30 - 60 watts is plenty for me.
Also, I would note that an Atma-Sphere OTL will put out less power into lower speaker impedance loads and more power into higher loads vs. a high current sold state amp that doubles power as the impedance is halved, but has reduced power as the load doubles. Ultimately I pay less attention to power these days. I'm more interested in how much distortion the amp is going to put out (it will put out less into higher loads) and if I can get my desired SPL level in my room with the amp and speaker of my choice. I find 30 - 60 watts is plenty for me.