Since both the M60's and M80's have a very linear frequency response, I would imagine they sound equally good at low and moderate volumes. The M80's will give you more bass extension (by a whopping 3 hz) and higher spl, but with your current electronics, I doubt you would be able to drive either model to their potential. Plus if your sub is good, it may be redundant.
Therefore, I recommend some new amplification to the M60's to see what they are able to do before deciding to switch up to the M80's. I was driving mine with a Denon AVR-989 which is quite similar to the 2309 and they put out lots of sound. They really are great speakers for the $$.
