Jim, the transformer noise is almost certainly due to the presence of DC current in your mains. I had the same problem when I switched from a MC402 to a burmester 911mk3, the torroidal transformers in these types of amps are more sensitive to a DC offset and will mechanically hum as a result. It's possible that one of your momentum's is more sensitive to this than the other or that they are on different circuits with one containing more DC than the other.
Things like washing machines, fridges, dimmer switches all add DC offset to the mains; when the AC average is not at 0. There are several solutions available including the PS Audio humbuster and in my case the burmester 948. The 948 has a DC readout and keeps the DC offset lower than 0.02V when conditioning is activated. If I deactivate it will jump up to around +0.60 V and my 911 will start to hum loudly.
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