How much hum is normal from a new tube amp

Know that I think of it I had a hum that was intermitant and it turned out to be a loose wire on a Cap. BE CAREFUL investigating that one.

* Now that I think about it

None. I have tube mono amps feeding La Scalas. No hum at all. 

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. 

Transformer connected to the AC line with the wrong polarity. Send it back to the builder.