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?
Check to see if the drivers are coming loose. just tightening the bolts or screws connectint them to the horns.
Or for the woofer, to the cabinet.
That may be all yo need.
Do not over tighten, bu they should be snug.
(Tight, like say as tight as a normal teenage girl could get them hard as she tried. Not tight as some guy pumping iron!)
I don't know, if that will help you, but I had similar, if not exactly the same problem with my Duo's (I thought), that turned out to be old failing tubes in my CD player output stage.
Don't necessarily assume it's a speaker.
I would also look at some other then the speakers....like overdriving the input on the amp from the passive......