The more I carry my M0 and A45 around, the more I see myself going back and forth between the two. In my opinion, they really are complimentary in the sense that the A45's more laid-back and dynamic signature lends itself really well to Rock and Electronic music when you're just on the go or at the gym. The M0, on the other hand, seems just a little more intimate and focused towards detail retrieval. My ears to feel a little more fatigued during long sessions with the M0, but I can listen to the A45 all day and still want to play more. As an analogy, if the M0 was a pair of studio monitors, the A45 is more akin to a home Hi-Fi system in terms of staging and frequency emphasis. As a result, I tend to use the M0 when auditioning my needledrops pre and post-processing, but the A45 tends to get the nod for all of my CD rips.
The amp on the M0, in turn, kinda wipes the floor with the A45. Granted, it's at the sake of battery life, but you really do have to crank up the A45 (for me, usually 75/120) to get the same output that I can with the M0 (30/100 in High Gain, 50-60/100 in Low Gain). Gain control on the A45 is also a little different in that, while the M0 uses a 0dB or +6dB gain, the A45 simply uses a 0dB or -3dB. A 2V output from the M0, in its form factor, seems really impressive to me.