First and foremost, try using another mic through the Bose without connecting the keyboard to the Bose. If this works, then there is nothing wrong with the Bose.

Now, about that hum. Do you have anything else plugged into the power strips, PC, etc... PC power supplies are notorious for causing hum problems because of conflicting ground systems. This can be overcome with a ground defeating adapter plug used with the laptop power supply.

Gary