The Mezzo's main strength was its midrange. Transparent, uncoloured and gorgeous within a slightly cool presentation. I used to think the advanced Teoxid inverted dome Focal tweeter was sweet and extended, but the Usher's Beryllium tweeter is frankly in a different class with incredible detail and extension but not in-your-face at all.
In the midrange they are closest but the projection and focus of voice and sheer neutrality of the Usher wins the day, not to mention the latter's serious dynamic punch.
No contest in the bass. The Mezzo was impressive for a single 11" driver per side, but needed a REL Studio II subwoofer to reproduce the bottom octave for my beloved organ recordings. No such need with the Be-20, which is a large speaker for my room and benefits from good room treatment.
Build and finish-wise, both speakers are superb.
Overall the Mezzos were always on the slightly cool side of neutral. The Ushers are as near as dammit neutral or very slightly warm in their presentation and better match my preference for transistor electronics.
