experience with MB Quart Pur speakers?

Trying to gather more information about MB Quart Pur speakers - including comments on performance, amplifier matching. I recently purchased them, trying to give them a fair trial with several different amps before deciding to keep them. They still seem somewhat harsh in the upper midrange and above and "shallow" in the lower midrange and below especially when recordings have vocalists or solo instruments playing. Is this the nature of this speaker or will particular amplifiers address this? Any speculations on the value of this speaker? Thanks for your help