As far as I can tell, the M-100 seems to have better scalability than M-80, that is, M-100 has a larger room for improvement when amped. Both M-80 and M-100 are highly sensitive, so they can still be driven efficiently by anything, even straight out of an iPhone. I own V-MODA's portable DAC/amp VAMP, and although I enjoyed the M-80 with it, I felt the improvement of an having an amp wasn't significant for the M-80. On the other hand, I can clearly hear a decent improvement with the M-100, comparing un-amped vs. amped.
That's good to know.