Check the air inlet: instead of aluminum duct work, you could try a corrugated soft duct. It may transmit less noise. And it wouldn't cost an awful lot. Is it for heat and/or airconditioning? A true audiophile would cut the ductwork out and freeze in the winter and swelter in the summer, as long as the tunes were sweet!

I'll bet slapping on an extra layer of 5/8" sheetrock (or drywall as the call it in the Midwest) would help, using Green Glue. (hint: I'm selling a case of it right now under "Accessories"). Sure, it would involve moving a bunch of stuff off your walls, etc. and painting and mudding the wall, but it might be halfway cost effective.
In my last room, the wall that faced our electrical closet was just made of studs and drywall, no insulation, no drywall on the other side. That situation drastically changed for the better when I got flooded last summer and redid everything.
Do you have drywall on the ceiling, or is it a drop ceiling? How about a few pics??? And do you have cans in the ceiling? I went to sconces (thanks again JLM) and it helps a lot!
Paul