I'm in the dark.......

If you have access to the fuse box for your apartment and if you are reasonably handy electrically, you could check to see if the grounds are even attached to the outlets. You could also try reversing the hot and neutral -- I've heard that sometimes electricians hook that up in an inconsistent way and it could cause problems. This is a long shot type of solution but only costs your time (do this only if you can turn off the power to the outlets you are checking or it could cost a lot more). Check some ground hum threads for more advice -- yes, much of it is lifting the ground (which does not work for you) or installing a dedicated line (which you can't do), but there are also other good ideas. Good luck.

Two other quick things. First, you might want to check to see if the fuse box connections are clean and secure -- if they don't appear to be, contact your landlord. Second, did you try the Creek at a friends' house to see if it might be a problem anywhere? Maybe the ground issue is in the unit (not real likely, if it sounds OK with everything else off, but it's another free test you can perform).

actually, in every appartment there are more unused circuit brakers or fuses than used. you can actually request an installation(from the management permission certainly) of another power line if you will explain your needs: state for instance that you can't use iron, A/C and microwave.
if it doesn't help you can install a dedicated line from your pocket.

I know this is really late, but I just came across this post now. Maybe this will help, somehow...

I had the same problem with my 5350SE. In fact, I returned two different units to my dealer because they were humming. The third one was fine for a few months, but then began to hum. It didn't seem to affect sound quality, but was really annoying, especially during softer passages or when using headphones.

I too thought the problem was in the wiring of my (rented) house. I plugged the amp into every different outlet, and it seemed to hum louder in some than others; in one or two it seemed not to hum at all. Worse, the hum wasn't even consistent; sometimes it would hum from the moment I powered on the amp, other times it would start to hum after being on for a while, and still others it wouldn't hum at all. But after a while the humming became more frequent.

I bought a Brick Wall surge protector/line conditioner. This didn't help with the hum, and convinced me that the problem wasn't in my house's wiring. I called up the dealer again (who claimed that neither of the amps I'd returned had hummed in the shop), who suggested that I do things like tighten the transformer screw under the amp, etc.--none of which fixed anything--so they passed me on to Roy Hall directly. Finally, he told me that some of the Creeks had faulty transformers, with too much iron in them (or something like this--I'm no electrician/engineer). So I took the Creek to the Service Bench in Norwood, Mass., and they replaced the transformer in an hour or so. Haven't had a problem since.

And let me add that I *love* the sound of the Creek. I routinely stay up way past a reasonable hour, listening to records on my Grado 60's, unable to make myself stop.