Amp transformer buzz/hum from DC on AC line?

DC does not make a humming noise in a transformer.

First, I assume you are talking about a mechanical hum from the transformer itself and not a ground loop type hum heard through the speakers. Those two issues are considerably different.

The sin of too much DC into a transformer is that it saturates the magnetic field. Enough DC can also damage or overheat the transformer.

The far simpler and more likely explanation is that the transformer with the louder hum either has a loose lamination or a coil that is not wound tightly enough.

Is the hum audible at your listening position when no music is playing? If it is not I wouldn't worry much about it. If it is audible, I'd call the manufacturer to discuss having the amp serviced.

I agree with Mlsstl, the only thing I might add is that the transformer is coupled to the chassis with some type of rubber grommet in most case's. You might try loosening the transformer and put a rubber wedge between the base of the transformer and the chassis.One of my mono amps has the same problem, but I can't hear it from where I seat.

I have measured .46 volts DC on the line to both amps.

How did you go about measuring the DC voltage on the AC line.

DC voltage on the line can cause transformer mechanical noise.

http://sound.westhost.com/xfmr.htm#7-noise

http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

http://www.altavistaaudio.com/hum.html#trafo

Are you sure your amps have the 60HZ transformers used in the US as opposed to the 50Hz transformers? Sometimes if a transformer is designed to see 50Hz putting it on a 60Hz line can cause hum.