Having done an a/b comparison of the 60s and 80s, I can tell you that you will not loose a whole bunch of sound. In an a/b comparison the M80s give more detail. I hate to use audiophile terms, but the upper end really is more transparent and gives slightly more detail on the M80 and there is noticably more bass. The upper end is a subtle effect though and a sub makes up for the bass.
IMO, for $300 it is a worthwhile upgrade. If you have to add another $700+ for a new receiver, I would stick with the 60s.
