I didn't care for M^3 all that much when I had it (with STEPS). In comparison with Presonus CS I thought both sounded roughly the same with M^3 provided a little clearer/less smeared sound, and CS had a little better bass and more forward midrange using the HD650. Ofcourse, Dynahi wiped the floor with both.
Dynahi did well with HD650 but not so well with Grados (the midrange is not that sweet and the high were, at times, unbearable)
336/332/337 is very good with HD650 but not so good with Grados (no bass, and thrill highs)
Corda Opera/Symphony is very good with HD650. Comparing to Dynahi, opera does not have as big soundstage, does not hit as hard/as deep; what it does have is a sweet and intimate midrange, which makes any music with a lots of midrange info (practically all jazz/classical/vocal) very seductive. The overall sound is more transparent, even though Opera/Symphony lags behind Dynahi in several aspects. Grado works surprisingly well with Opera/Symphony. No more shrill highs, what Opera/Symphony gave up in the deep bass slam through Dynahi is more than made up with the sweeter and more transparent mids. I feel Opera/Symphony is like the middle ground between all tubes (DV amps) and all discrete SS (Dyna-lo/hi)
I know this does not answer your question, but now you know M^3/Dynahi/lo (e.g. G-lite; GS-1) is not no-brainer that some here are preaching. Buy used, listen for yourself, and make informed decision based on what you think sounds good.