If you are using a 4 channel amp most probably you are using the channel 3 and 4 bridged to power the amp, most amp may LPF sa channel 3&4 set it to 60hz. then listen if the voice is still there? adjust it lower gradually.

dati ang old setup ko may naririnig din akong faint voice sa sub, now I change my amp and Sub all I can hear on my sub is Ummpp.. I guess some voice frequency still passing to the filter of the amp.