Coincident Partial Eclipse or Victory (if used) would fall nicely into your price range. They are fairly sensitive and have very OTL friendly impedances. I believe Tonian Labs also makes some high impedance models, but I am not very familiar with that marque. With the right sepaker, I think you'll be surprised how much amp the M-60 gives you for a reasonable price.
What speakers match well with Atma Sphere?
I'm considering the switch to Atma Sphere M60's tube mono blocks. As much as I love my Pass Lab X600's, as a renter who has had to move three times in two years, they are a pain to move. However, my current speakers are Dynaudio Contour 3.3 (4 ohms) which require a lot of power. My understanding of the Atma Sphere M60's are they perform well with 8 to 16 ohms speakers. So what would be an excellent speaker in the $3000 -$4000 (used) range?