How much hum is normal from a new tube amp

OP, Good you sent it back. I would not accept any hum.

Lab12 is a good company and their products sound great.   The Music Room is a safe bet when you have a problem.   I just purchased a Cyrus integrated amp from them.   I had an issue that led me to believe the unit was defective.   It wasn't,  it was a simple problem but they were great to deal with.   Hopefully you get it straightened out.   

While I’ve found the KT-150 Tubes to be the least reliable power tube I’ve ever used, they never produced hum in my amp (primaluna evo 300).

Agree with the choir to return the amp for a refund or replacement. I had a similar issue with a Carver Crimson 250 amp and was lucky to work with a dealer who fully refunded the unit. Best of luck with your replacement.

I have very recent experience with this. I've had a Pass XP10 pre amp running my Pass 250.8 through a JL Audio CR1 active crossover. I have a purpose built very solid, very tight and quiet listening room. I could hear a very faint hum at 7 am when the house was totally quiet, otherwise I never heard it. I just upgraded to an Audio Research LS28 tube preamp - surprisingly it's dead quiet !  

Chances are a replacement of the same model will exhibit the same issue.