How much hum is normal from a new tube amp

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. 

The humming is most commonly caused by the magnetic field from the coils causing the iron core of the transformer to change dimensions--as it does so, it is vibrating, hence the noise.  To some extent, all power transformers vibrate this way, although most are designed so that the vibration is minimized and sufficiently damped.  If the noise is not too objectionable, you should just ignore it--there is probably not anything wrong with the transformer such that it will get worse.  In any case contact the manufacturer if you can about this issue to see if it really is a manufacturing defect that they will address.

Someone above mentioned checking the bolts that hold the transformer down to the chassis.  This is a very good first step.  If they are loose, the vibration from the transformer might be made worse by the poor coupling to the chassis.  If you are not experienced with tube gear, have someone who is experienced go under the hood because there may be high voltage stored in the capacitors that you don't want discharged through your body (amps can shock even when unplugged).  

Good luck.