What is the midi control set to do for your digital board?
I think it's in the text, but at any rate, MIDI travels through the cat5 leading to stage (next to the audio) and controls the gain of the preamps on stage.
There is an Appsys RGC (Remote Gain Controller) in the preamp racks, which receives MIDI and puts out a proprietary Appsys signal that in turn controls the gain setting of the preamps. The preamps have been modified slightly to be able to receive the Appsys control signal.
Appsys claim they can provide mod kits for 90% of all preamps out there that have an ADAT output.
I'm working on controlling the preamps directly from the desks, but have been reasonably happy with the gain-controller on top of the desk so far.