Amp transformer buzz/hum from DC on AC line?

I have two tube monoblocks with EI type transformers.
One amp hums much more than the other. Not through speakers. It gets somewhat louder and much warmer than its counterpart over time(3-4 hours).
They are on different circuits, I have tried swapping circuits. I have isolated it by turning off every breaker except the one to the amp, so it's not from somewhere else in the house. I have swapped all the output tubes from one to the other.
I have measured .46 volts DC on the line to both amps. Could the one be more sensitive to DC than the other?
Is this enough DC to cause a problem? I can get a PS audio Humbuster on free trial to make sure, but it sounds too good to be true. Any thoughts?
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.
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.
I second what Jea48 said. I have deliberately induced hum in transformers with DC as an experiment and solved a number of DC problems with simple DC blocker.
How to measure DC on the AC mains....
As of the last while, I had a buzzing/humming sound coming from my Plinius 9200 integrated. Fairly loud - I could hear the sound from 5 or 6 m away in my listening room - but didn't seem to affect the sound from the speakers. There was always a very slight buzz from the speaker, but you had to hold your head pretty much up against the speaker with volume at least 3/4 (with no input!) to hear this. I attributed it to normal operation.

During my early period of owning this amp, same thing happened. Conctacted the Plinius rep and had a replacement torroid supplied for replacement; Plinius acknowledging there were issues with an early run of the torroids. Solved the loudness of the torroid, with a minor but still present buzzing persisting. Thinking my recent problem may be a hardware issue again, contacted Plinius. This time, they suspected DC on the AC line. While somewhat suspicious, for no sound reason other than what occurred the first time, I decided to start checking this out.

Long and short, this thread among many, arrived at the suggestion of using the Humbuster III to handle any DC (if any present). Picked up one and it arrived today. Put it in line with the Plinius, sparked the system up, and no noise whatsoever. I had to put my ear against the amp to confirm the status 'on' light. A couple of hours of listening to some familiar material, no veiling or change in the sound of the music. In fact (and I think this is my mind at play) I thought there was a slight increase in detail and ambiance in some of the solo piano recordings that I had been listening to. And I should add that slight hum in the speaker that I attributed to normal operation is no more.

For $200-odd bills, an incredible device.
An update to my previous post.

Listening to a wider spectrum of music while using the Humbuster III, the amp still is dead quiet. But I have also found that the unit removes a very slight amount of detail from the music, what I'd call a slight reduction in higher frequency. Most notable on the leading edge of acoustic string, voices. And by this it affects the definition and air about the same.

I'm going to continue playing for a few months and see if it's a case of needing burn in (always suspect of that, but had a good example of it a while back with an upgrade of power cords and better IC's). We'll see. Nice to see the hum removed, and the amp is running cooler, but not happy with the cost of doing that.

Someone had mentioned in this forum something about this unit that was contrary to what was thought as it's sole efffect i.e. DC removal. The documentation with the unit states 'over/under voltage surge protection' so I'm not certain just what goes on within and if something beyond the DC removal contributes to this attenuation of detail.
Do we still think the humbuster filters some higher frequency detail from the output of the amplifier?
One thing I can say is that the humbuster works well on toroidial type transformers, but not really on EI types. There may be some exceptions, but that's the effect I have noticed on a variety of amplifiers and other gear.
Are you sure it's not related to a dvr motor running? Not sure if you have a dvr but they can cause a racket if not isolated.
If your amps do not have capacitors blocking dc at the input, then the amps will amplify dc and you will hear it on the output or the amp's dc protection circuitry will shut it down. It sounds as if you have excessive dc offset on the amps. or from the pre-amp. if the amp does not have a dc offset adjustment, then it is possible that a device in the amp has or is failing causing a dc offset. you would have to open it up or take it to a qualified tech to check each component for correct operation or failure and replace/repair as necessary. If you disconnect the amp from all other equipment,plug it in and turn it on and the dc offset is still there, then it is the amp and must be repaired. amplifiers that are well designed will amplify any signal, be it ac or dc. dc at the input will cause big problems if the amp doesn't have dc protection circuitry. Such as blown amps or speakers. or if it does have such protection, it will simply shut the amp down until you find and fix the dc offset problem. If your power supply filtering isn't up to snuff, the dc on the ac line may find its way to the audio amplification path. So, if this is the case, you may want to check the power supply capacitors and bypass capacitors to see if they are leaking, going bad or failed. Or add some bypass capacitors if they aren't any there. DC on the ac line shouldn't be there. But if you can't find the source, then an ac isolation transformer or a power line conditioner is necessary.

I have two tube monoblocks with EI type transformers.
One amp hums much more than the other. Not through speakers.

It gets somewhat louder and much warmer than its counterpart over time(3-4 hours).

Mechanical vibrating noise from one power amp's EI type power transformers.

My bet is loose laminations.....