have you thought of using noise cancellation techniques to get rid of the hum?
now this might sound like over kill or rocket science but bear with me.
get a mic to feed the noise from your mac into your mixing desk, phase invert it and feed it back into your speakers. The theory is that the noise will cancel out.
Have a look at the sennhieser noise cancelling head phones. They are great for low level hum like that on an aeroplane and the same principles can be used in to get rid of the Mac noise.
Hope that makes sense, if not then I am sure that there is someone here more articulate than me who will put the record straight.
"Never put a recording studio in your home, it is a black hole into which you can pour unlimited amounts of money" Pete Waterman.