Having heard the M60s, MA-1 mkIIIs, and MA-2 mkIIs, I think its fair to say there is not much sonic difference between the Atma-Sphere amps, assuming a reasonable load. I suspect the choice depends more on speakers, room size, the 'scale' of reproduction one prefers, and one's priorities regarding authority and weight in the very lowest frequencies.
The MA-2s coupled with Wilson Maxx 2s (in a setup v. similar to what Marc has) are *very* impressive. Not quite the very last word in lowest-end heft and punch as one might get from say, the ARC Ref 210s or the CAT, but little else is lacking. The MA-1s and MA-2s certainly offer top-notch low-end tonal definition and articulation. And while there may be equals, I've yet to hear any amp that is quicker than an Atma.
In terms of tonality, the Atmas compete with the best. Until I learn better, I cash out the issues of 'lean' or 'full' as a question of relative higher order harmonic distortion. The issue of tonal 'weight' is one I gauge as a matter of personal preference.
And, fwiw, choice of driver tubes definitely make a difference here. Most all Soundstage reviews stick with stock tubes - and rightly so. The Chinese 6SN7s that Ralph provides are some of the best current manufacture in terms of low noise, construction consistency, and reliability. But put in a few Ken-Rads or RCA 6SN7s, and then tell me what you think about harmonic weight.
While lowest-end authority may (very) slightly tip in favor of beefy transformers, on the flip side, I've heard nothing that competes with the exquisitely filigreed top end from Atmas. It just keeps going and going and going.
In the end, at this level of quality, their is no 'best' other than what works best in your system to yield the level of sonics and satisfaction that work best for you.
Lastly, I can't say enough good things about the new MP-1 mk.III w/ phono in conjunction with Atma-Sphere amplification. (Thanks to those who read my review.) Certainly at its price point. I'd enjoy reading comments of comparison about other preamps folks find work well with Atma-Sphere amps.
Tim