Preamps for Atma-Sphere OTL Amplifiers

Can't give you a specific recommendation, but based on my experience, you should limit your search to those with true balanced output.

I used the mk.IIIs with a CJ ACT2 for several months. Its an excellent single-ended preamp that matched nicely with the M60s, which did nothing to hide its character. Later sold it to buy an Atma-Sphere MP-1. I'll concur with Swampwalker on using a balanced preamp.

An ARC Ref3 could work well, but I haven't heard it with the M60s. Are there sonic issues that lead you to look for something different than your MP3?

For what it's worth, there's a review at 6 moons that liked the MA-1's with a Shindo pre.

I can't possibly imagine a better pre than an Atma-Sphere MP-1 or MP-3. I just went from a REF1 to an MP3 with my M60MkIII's. The switch was a revelation. You don't mention what options your Atma-Sphere gear has. If you don't have the Teflon coupling caps in both along with the regulator option in the MP-3, you might be better off with those upgrades for a sonic improvement. I should also mention that ultra low noise perfectly matched 12AU7's in the MP-3 along with low noise matched 6SN7's in V1 & V2 in the M60's will make an enormous improvement.