As far as low volume performance, I had not considered that. I have not heard M60s so I dont know first hand how they would compare to the M80s. I do however find it difficult to believe that there would be a noticeable difference between the two speakers at the same DB when using a sub. What is it about the M80s that would make them perform better than the M60s at lower volume?
I have done some rudimentary blind testing and when A/B'd the difference was noticeable to myself and Mojo. The 60s are just a bit more laid back in the upper ranges, which to my ears sounded ever so slightly muffled/compressed. I would bet you would prefer the M60s over the M80s you have since you run the 80s with the resistor. I found once volume levels increased, say 85db and up, the M60/M80 differences were negligible. The 60s seemed to open up and become more spatial.