I'm with Rushton, I don't expect to ever try any other amps as my M-60's work so well with my Köchel K-300 horns which I don't plan on changing either. Unfortunately I can't speak to the question you've asked. I can say that Atma-Sphere preamps offer truly outstanding performance.
Balanced preamps with Atma-sphere MA-2's
I just purchased a pair of Atma-sphere MA-2 Mk. II.3 OTL monoblocks (thanks Siddh) to drive my Dynaudio Confidence C2's. Right now, I am using a single-ended SAS 10A tube preamp with pretty darn good results. But, Ralph at AS and Brian at Essential Audio state that the AS amps really need a balanced preamp to shine. What are you MA-2 owners using for a preamp? TIA.