Further to my reply, if you had left the bass management of the Denon set on Small for the M80s, and forgotten to reset it after the ep500 was removed, the M80s would sound thin and lacking deep bass. The sound would likely be perceived as underpowered because deep bass certainly adds to the sense of power and loudness in sound quality.
There are other factors that might also contribute to your problem, but first check the bass management and report back.

Alan Lofft,
Axiom Resident Expert (Retired)