A brief addition. The Equitech website may be of help as it offers quite an overview of power problems. If not you might give them a call. I found an Equitech 2Q solved my hum problems. Their Balanced transformers are quite different than regular isolation transformers. Costwise you could try one of their "blowout" units, if they are available.
Why do my transformers hum sometimes?
The power in my home theater seems to be especially bad. In order to improve things, I'm using a Monster AVS 2000 voltage stabilizer for my main components, an additional filtered power strip for the front end components and an isolation transformer for the subs. At times (very frequently on Saturday afternoons), all of the transformers in my system hum audibly. Anybody know why that is? My guess is that the 60 Hz AC sine wave becomes distorted somehow but I don't really know.