Chris....I hear this too. Its almost like theres a bit of ground loop somewhere in the system and I've been known to unhook the entire studio and test every cable searching for it never to really solve it.
For anyone who has or does work in an office are with a lot of flourescent lighting, especially older flourescents without solid-state ballasts, you are being subjected to a low 58 cycle hum at all times. Its a Bflat in case you're interested. The reason its not 60 cycles like the power grid operates at is because of the ballasting.
Okay, so I see what you're up to with this.
Perhaps something along the lines of a Manley Passive at the gateway to the 2 buss will add enough of its own set of harmonics to filter out this thing. Big tubes, big iron, nothing gets out without it becoming what is dictated by the circuit itself.