I also own M-60s, and I was fortunate enough to score a deal on Ralph's MP-3 which, unsurprisingly, is the best of all the preamps I've tried. Prior to this my favorite was the LTA MZ2, which bested a Modwright LS-100, a Don Sachs Model 2, and a Decware CSP-3, although they were all very good. I also own a Bifrost (not the 2) and I've been very pleased with it, but I'm 90% vinyl so it doesn't get a lot of use.
If you end up with a single ended input to the M-60s, make sure you have the jumpers for the XLR pins 1 and 3. That made an audible difference with some of the preamps.