Avantgarde Duo G2 ? sound problem

Hi, Forum, I have a problem with these speakers; the lower octave of its midrange around male vocals produces a sort of harshness or distortion simular to something that is overdriven with high input signal just not that obvious(not the case here). My anp is a Sherbourn 2-160 ( good match for the speakers by the way) and preamp is passive Promitheus TVC. I swapped equipments for trial but the problem remains. How could I remedy this?