How much hum is normal from a new tube amp

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. 

It's down to the electromechanics of the transformer.  These things can be a bit elusive in that the amp might hum less in another installation. But it's still indicative of less than perfect design and manufacturing.

I run Kt150s in my Vac amps and they are dead quiet.  Agree with return

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

                 ^^^^^ What he (et al) said! ^^^^^