Stain, Don't get me wrong the M80s do have have a thing or 2 over the M60 and I would be very surprised if you didn't think the M80s sound great even with MP3s, you will find that they sound even better with a more full range source, the actual cd or higher bit rate MP3. I just think we may push people over their budgets to get the M80s, when they really would have just been as happy with the M60s, you never know what you are missing sort of thing.
A few years ago most talk here was of the M60s, then upgraditis hit the group and the M80 has become the defacto standard most of us have.
So far as paying to return the M60s, you would have only paid the difference in price between the 2, shipping both ways would have been free.
Anyway from what you have described as mostly lower volume listening, the M80 IS the speaker you want.
