So many things can cause a ground hum. Cheap power strips, running power cords too close to microphone or signal leads, house wiring issues and other things on the same breaker, causing issues, like light fixtures or similar. New space, you could have missed one of these wire issues, if you haven't already looked for these.
If your electrician has all hot and neutral wires correct, have him check for other things on the same breaker. Moving your music room to its own breaker, if you have one, could be good. It's good that you have a professional doing this, as it's so easy to make mistakes, if you aren't one.
A separate ground, as was already suggested, is also a possible fix. But, if these amps were all doing OK in the previous room, it does seem strange, if the whole house is on the same ground. It sounds more like something else is on the same breaker. But, even if it doesn't fix this issue at hand, giving this room its own ground would be a good thing to do, if it's not going to cost you too much.
It is strange how electricity can cause such things, when it doesn't appear like anything is really different. Check things like wall warts, since many of them only have two prongs and no real polarity blades. If you can, unplug all of these, to see if the hum leaves. If it does, plug one it at a time, to see which one is the problem. Another thing to try is to flip those around, one at a time, to see if one of them is causing an issue. If you check one and the hum doesn't change, put it back how it was. Check each one, in turn, to see if one is maybe causing a hum. Of course, if one of these is powering the unit that is humming, it might be the first one you want to flip.
Always use good quality power strips. A $10-$15 6 outlet strip isn't good quality. It will work fine for powering lights, but in a studio, it can cause problems. Also strange, back several years ago, I had all of the bulbs in my studio incandescent. To save a bit of cash, because incandescent bulbs were difficult to find, I changed them to those pig tail bulbs. I was told they wouldn't cause a problem. They did. But, instead of having to replace every bulb with incandescent, I just had to change out one and the hum went away. Electricity is sometimes pretty strange. Now, they're all LED. No problems, there at all.
Hums are annoying, I know. I've changed out all of my power strips and thought everything was better than it was before, but I ended up with a hum. It looked great, but sounded like Sh*t. Talk about annoying. Sometimes, it's just a process of elimination by turning things off in a sequence, to see if something changes or removes the hum. Sometimes, it's a combination of two units that are causing the hum, each alone, not humming. Moving one to another power strip could be all it takes to fix that.
Good luck. You'll find it, and the satisfaction of silence will be near overwhelming for you. You'll have a smile on your face for days.