Buzzing sound coming from my amp

I have a Krell KSA-150 amp that's probably about eight years old. I bought it used about a year or so ago.

when I turn it on, I hear a low buzz (barely audible, you have to put your ear to the speaker to hear it) coming through the speakers. The amp is plugged into a power strip that's connected to a switchable outlet.

I use XLR cables between the amp and preamp.

I've been told that it's better to plug an amp directly into an outlet, rather than a power strip, and perhaps not a switchable outlet. I tried this and no difference in the sound.

I'm sure there could be many reasons for this noise, but I was wondering if someone could offer some advice for tests or maybe your knowledge of amps that could help to pinpoint the cause.

Or, is the cause likely in the amp itself and not some external issue with the electrical in my house?
Most low level buzzes are caused by a grounding problem of some sort. It is frequently caused by a ground loop (where there is a difference in the ground at two different points on the system which causes current--and buzzing on the ground). You can try using cheater plugs on different components to determine whether or not you have a ground loop problem. While it's not a good idea to use the system without proper grounding (especially high current equipement), you can determine if this problem exists on your system. There are other ways (besides cheater plugs) around it. These are more complicated and involve placing resistors and diodes on the ground runs between equipment--this way you can control the current flow and keep the noise away from the equipment. Hope that helps
The sockets in your house could be installed with reverse polarity.
I have a very small hum coming from my speakers. I tried a cheater plug and the buzzing got much louder! What could this mean?
Using cheater plugs is a METHOD to test to see if you have a ground loop problem, not a solution. Your best bet is to plug everything into one outlet/line. Or, you may try an isolation transformer.