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.