I have a manual here for the Souncraftsman MA5002 amp. There is a half page of info on ' Transient Intermodulation Distortion. T.I.M. And the test method proposed by the AES.
I'm not sure if it's of any interest. But you may try searching T.I.M. if it is.
If you get a chance, wing me the approx time-stamp of that voice IMD you heard. I played through the track on BoobTube but couldn't find an obvious section.