How much hum is normal from a new tube amp

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.

If the hum level changes from time to time, it may be from  DC offset in your power line caused by other things in your home contaminating your power.  You can try powering the amp from a different circuit using extension cords, or turning off each circuit until the hum stops.  Depending on when the offending equipment is in use, the hum level will change which may be an indication that the cause is in your power delivery to the amp.  Some transforms are more susceptible to this problem than others, which may account for why this is only happening to this amp.  Toroidal (round shaped) power transformers are more apt to hum from less than ideal power feed.  The problem with DC offset is that one cannot easily measure it to get a definitive diagnosis and most power conditioners don't deal with it. 

Another possibility is that the power transformer is overheating because of a problem in your amp or because the manufacturer put in one that is not up to the task.  Is the transformer hot to the touch or merely slightly warm?  If it is fairly hot, you should talk with the manufacturer even if you had decided you could live with the slight hum because there could be a bigger problem.    

This is the right thing to do, particularly if the manufacturer is not giving you grief.  You should not have to live with loud noise and doubts about your gear.

Good luck.