Using sensitive headphones like Grados, my SQ-84 hums/buzzes maybe 70% of the time at home. I tried using the amp at work, and there's no hum/buzz at all. So now I have a 30-pound tube amp sitting on my office desk.
I guess dirty power is probably the cause of the problem at home. The power choke is close to one of the output transformers, which is probably why I only get the hum/buzz in one channel.