How much hum is normal from a new tube amp

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.

Might be a lemon set.

i have Lab12 Integre4 MKII, zero hum.

The demo set I auditioned had a slight hum when using XLR input, but none when using RCA input. My brand new set has no hum though at all. Perhaps try switching the inputs first? If it still bothers you just exchange a new one. 

Now that I listen to it more, I think the sound is coming from the inside. I'm not very technical and I don't know what the implications of that are. The hum is consistent. It starts within 8 seconds and never wavers. I suppose sending it back is the best course.