Hum Cancelling Circuit
My first concern would be what would happen if you had two channels. Remember that the ground is usually common. If you try to separate it in one side, it will most certainly be joined on the other. Then your two circuits will be fighting one another, which I suspect would make things unstable. 

My second concern would be what would happen on the corner frequency. Have you tried an AC simulation? 

My third concern is (an this is just a hunch) common mode gain. I haven't thought it through, but this reminds me of a differential amplifier with imbalanced resistors and those exhibit common mode gain.