I've tried a number of different isolation transformer brands and had experience with DC on the mains. The 2,000VA transformer in my first post in this thread buzzed like a gas powered chainsaw in time with the agitation of the household washing machine anytime it was running. Equi=Tech Q type and Plitron LO-NO transformers are designed to be quite immune to DC. Simple DC blocking circuitry will solve that core saturation issue and noise. I've built a number of DC blockers for people that either totally eliminated transformer buzzing or reduced it to a tolerable level (see post dated 8/4/09 at link below). PM me if interested in circuit details.

AudiogoN Forums: Do all toroidal transformers hum?