Permanent 50hz

It actually sounds like a minor ground loop hum.  If you have to get within 5cm of the speaker to actually hear it, then don't worry about it.  My system has the same thing.  Sometimes this will be very noticeable in systems and people can have a very hard time getting rid of ground loop humm.  Humbuster may or may not fix this.  Sometimes, you'll need to get the Granite Ground Zero to resolve this ($500+)

“Why would a 50 Hz tone be coming from the mid range driver?”

Excellent question.

The same reason midrange information comes through the woofer. Why do you think the woofer dust cap rings and buzzes like crazy?