Reading this thread I realized there isn't a whole lot of information about how the amp actually sounds, especially compared to other amps.
My own very, very brief impressions:
Compared to the M-Stage, I found the SA-31 faster, brighter (neutral?), with a larger soundstage and much more powerful, yet controlled, bass. I preferred the M-Stage with Denons, but have preferred the SA-31 with everything else (particularly HE-500, T1).
Compared to the Bottlehead Crack, obviously the SA-31 is much, much better with the HE-500s. In fact, I find that to be a very good, highly recommended match. With the Beyerdynamic T1s I prefer the Crack. It sounds a little bit more open, a little bit more detailed, but still in a warm-ish, fluid way. Still I sometimes leave the T1s plugged into the SA-31 for a couple of days and find that I enjoy it a lot. Again the bass is very powerful and controlled, and the upper treble a little bit easier on the ear (the upper treble can get tizzy with the Crack + T1 for me).
Overall, I think the SA-31 a very versatile amp, brighter and faster and more dynamic than the M-Stage, but still a sound with great body and sweet highs.