I have the M80 which I set as small at the start. When I experimented on setting them as "large", they sounded better for me. Now, my settings are:

M80 - Large
All others - small
Sub - LFE (The LFE+main sounded too boomy for me)

The March issue of Audiofile suggested that you experiment on M50, M60, and M80 by setting them to "large".