Mechanical hum is mostly from a poor grounding layout. Sometimes it's caused by heater wiring and sometimes by bad transformer construction. I have solved a couple of tube amp buzzing problems by simply moving wires with a wooden chopstick. You shouldn't have to. I agree it should be returned because it's either a factory lemon or, worse yet, a poor design or construction -- which will definitely impact sound quality.
How much hum is normal from a new tube amp
I just bought a great sounding Lab12 Suara KT120 tube amp. The issue I am having is a hum that is coming from the transformers. I can hear it with no music playing faintly from my listening position 12 ft. away. The question is, should I return it, or is this normal acceptable.