Do you have an SPL meter?
It would help to know at wich level it starts doing it.

The in-room sensitivity of the speakers is only 87db with only 55Wpc of amplification.
So depending of what you are trying to achieve as levels, you might just be overdriving them (what you describe sounds like the amp clipping).
Try with an active sub (or at least without the Ep0).

See Mojo's signature