You should watch a show on HGTV some night called Holmes on Homes. It is a Canadian Production, but to get to my point, he is pissed at minimal code requirements and contractors who use them to save money. hence the cable runs.
But, just because your house is newer doesn't mean some contractor didn't try to save a few $$ and time is money.
But, on issue I also had a hum from a HK sounded lie a bunch of bee's they told me it was my house wiring. I sent it back not once but 4 times and still had hum.
Finally they replaced it with a newer one and hum dissapeared. and I practically re-wired my living room and half the house.
try another amp at your house or take yours and the speakers to another place.
And the ground is a safty issue big time if a hot gets lose at least it will trip the circuit breaker. If the box and even the recepticle is NOT grounded and a hot wire gets lose who ever touches the appliance box just because amplified
