I also have this problem.  I noticed that if you are connecting 1/4" unbalanced to a mixer or amp without a 3 prong chord you get the open ground noise, if you connect to an amp with a 3 prong chord or if you connect the XLR balanced the hum goes away.  So even if I am not using the XLR, I will sometimes connect it to get the ground.  Does the job for me, don't know if that helps you or not, but something to try.

Thanks

Dave

From everything you've said the problem would appear to be a less than good ground rather than a ground loop as such.   Electrically I'm sure the ground in your mains wiring is 100% safe, but in terms of audible hum caused by a tiny amount of AC being induced in the house wiring ground wire by the regular AC cables running in close proximity to the ground wire, it's not being grounded as well as it could.

At home I have zero grounding issues with my HD500 and any guitar.  However there is another location I go to regularly and the grounding in that property is not as good, so I get a bit of hum which goes away when I touch the strings on the guitar.

In terms of induced voltage we are talking about probably a lot less less than 500mV, which is nothing in electrical terms, but in audio terms that can be a lot

You might be able to improve matters at your property, by getting an electrician to install an earth 'spike' in soft, damp ground outside, and then to make a connection from this to your property's electrical ground.   The problem with a lot of modern properties tends to be a lack of underground metal pipes as would have perhaps been the norm in days gone by, which would have normally been bound to the electrical ground. Modern day utility services like gas and water tend to be supplied to your property using plastic pipes these days.

Nick