You it must be your amp. I am using a Kappa 7.1 as the center channel in my home theater powered by a Carver Silver 9t monobloc at 900wpc @ 4 ohms with the speaker set to full-range large in my home theater. No problems with the speaker as it is subject to a serious onslaught of sounds throughout the spectrum and it hasn't missed a beat yet.
If you bought the speakers second hand, the original owner must have changed something. I have owned all of the Kappas from 7 thru 9 and they have all performed yeoman duty as long as they have been fed good power.
It has gotta be your amp or your Kappas have been tampered with.