A long time ago in my 'not so bright' days (...I'm making a far reaching assumption that I've changed ;) ), I had a hum issue. I did the same thing, brought it to the store and no hum. Brought it back home and re-set it up. Hum. I set it up exactly the way I did before I went to the store.

'Exactly' meant that I went home and set the effects up the same way (a Turbo OD and Wah)....That is to say that, again, I set them on top of the amp. I used the wah but when practicing; most of the time, no. I put them on top cuz my room wasn't that big.

You see where this is going...I took them off the top to look at the back (while plugged in) and lo and behold....Yes, you guessed it, no buzz.

Moral of the story, take everything away from the amp like clocks, tuners, effects and just plug in to the amp and stand a ways away with the guitar. Buzz still? Also, if it buzzes when no guitar is plugged in, sometimes it's the cable. If it's an old house, is the outlet a grounded (three pronged) outlet?

This kind of stuff. Just throwin' it out there.