So today I thought I would try the simple solution - just stick in my home theater cabinet and shut the door.
First problem - it's too big. I have an Antec Fusion case so it looks like a home theater component, but it's just a bit bigger than any standard amplifier, and my cabinet is standard size. Annoying, but that's not really a big deal because -
It's going to get way too hot in there. I figured I would have to cut air holes in the back of the cabinet anyway, so I'll just cut a bigger hole so that the oversize case fits. I would up cutting just a few holes, since I had to use a steak knife to punch the entry holes and then my jig saw to cut them. (I would up breaking the steak knife blade, damn shitty stamped blades).
Now it's been running in there a few hours, and it is indeed much quieter, but it's also getting boiling hot in there even with the big air holes. Damn. What I need is a large solid box with two open ends, so that the sound is muffled but it still gets plenty of air.
My next thing to try is replacing the PSU with a lower-power quiet one. They even make fanless PSU's now though that's a bit scary because it just means they pump more heat into the case.
My other option is to throw this damn thing out the window, burn down my apartment, ride my bicycle around the world and never touch another damn piece of electronics again.