I saw this posted over on Audio Circle where a member was having problems with equipment hum issues. Thought the approach is well reasoned and smart, so I'd thought it wise to share it:

"Electrical Engineer here.  I have extensive experience with power related issues, albeit in industrial environments. Just to be clear I assume you are saying the humming is coming from your electronic devices and not your speakers.   I have a few suggestions you can try.

1.   Take your suspect devices to a friends house or work and see if the devices hum there.  If the hum remains the problem is probably in your gear and not with the power in your house.  If the hum goes way then there is most likely a problem with the power in your house.

2.  Find somebody who is qualified (like an electrician) and owns a DC voltmeter that can be inserted into your power outlet (PLEASE BE CAREFUL!) and see if DC voltage if present.  If DC voltage is present then you have some digging to do.  

3.  If you have a DC voltage problem, have your electrician go to your main breaker panel.  Open panel and measure for DC voltage between each leg and neutral (I assume you have 240/120 VAC service).  If DC voltage is present at the main breaker panel then turn all the breakers off (only if you can turn everything in the house off without endangering anybody).  If DC voltage remains then the DC offset is coming from the secondary windings of your utility transformer (which is probably shared with neighbors).  At this point you should get your utility provider involved.  The problem may be coming from a neighbor you share the transformer with.  If DC voltage is never present in your main breaker panel and you are measuring DC voltage at an outlet then the problem lies within your house.

4. Harmonic distortion problem.  The voltage waveform in your house should be a pure sinewave.  If it is a distorted sinewave you have to figure out why.  THE MOST COMMON CULPRIT ARE LIGHT DIMMERS.  If you have light dimmers get rid of them or keep those lights off or undimmed while you listen to music.  Devices with switch mode power supplies (like computers, tvs) can also pollute your power.  If possible try to run a dedicated (not shared with any thing else) power circuit to your listening room.  Some electricians will have power analyzers that can measure harmonic distortion.

5. If you dont want to try 2,3,or 4 try an electrostatically shielded isolation transformer such as Trip Lite IS1000 - good for 1000 watts.  You can try the IS1500 or IS1800 if you need more capacity. Available and returnable on Amazon. Plug the transformer into your wall outlet and plug your audio gear into the outlets built into the transformer.  The transformer has primary and secondary windings magnetically connected, but not electrically so it cannot pass DC voltage through it.  The electrostatic shield also collects and filters electrical noise.  Beware that isolation transformers themselves hum a bit and that may be a problem for you.  I use one of these devices located next to my equipment rack and cannot hear it from my listening position.  Some audiophiles will tell you not to use isolation transformers as they can impede or choke current flow to devices that draw a lot of power (mainly amplifiers).  I personally dont see this as a problem for my gear but YMMV.

6.  Somebody recommended a PS Audio power regenerator.  I personally have not tried one of these, but the science is sound.

For the record I don't work for a company whose products I mentioned.  Sorry for writing a book, it's the way our engineering minds work."

Hope this proves helpful.

HK