Remove ground loop hum by 180º phase shift?
Trying to address power line hum with a notch filter in the signal path is a kludge at best, and is unlikely to yield the results you expect anyway.

The way to address this is to deal with the real problem of how the power line noise gets into your signal in the first place.  Surely there is much out there on this issue already.  Go study it.

This isn't the only possible cause, but one of the common ones is bad grounding between equipment.  You probably have a ground loop somewhere, so that the signal ground in one or more places is carrying some power return current too.  If all the equipment is in the same room, plug it all in to the same outlet.  If it's dispersed, then you may need to look into isolation transformers and the like.