If switching the ground switch on the plinius to open solves the problem, then you already have your answer. Just leave it on open! No need to make it more complicated than it is. Whether the ground switch is set to the chassis or not doesnt' affect whether the signal is transferred as balanced signal or not. That comes from the design of the gear - and likely both are truly balanced pieces. If it sounds good it is good - leave it at that. And if there is a ground loop hum it's more likely to do with the lines in your house than the gear per se.
-Ed
-Ed