For a start, the Elevator EXP doesn’t hum. Transformers do. To be correct, the transformer itself doesn’t hum, but the magnetic surrounding fields easily get induced into it.

These magnetic fields are predominantly mains-borne. There are a number of hum sources including all your mains powered equipment, surrounding mains powered products, and even the wiring you see, and the house wiring you don’t see – all emitting magnetic fields by differing amounts.

The SUT, being a transformer, is influenced by them all. The result is hum. No matter how well shielded its case or how well you carefully position it, in most cases that isn’t very successful.

The Elevator EXP is an amplifier – it doesn’t contain a transformer so it’s harder for hum to be induced into it.