MRM, especially considering the sub that you have, M60s should be more than enough for the size room that you have. Regardless of whether you get the M60s or M80s it's likely that your 657 has more than enough power, so you shouldn't "think" that you'd need supplementary amplification. Unless the 657 would distort audibly or shut down, extra power would be simply unused excess capacity. More power simply means more maximum loudness capability, there are no other benefits that would mysteriously appear.
If you use your QS8s to reproduce ambience from classical CDs, using DPLII, you'll probably find that this significantly increases your listening pleasure.
