How much hum is normal from a new tube amp

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.

The whole system was fine. Only thing changed was the amp which HUMS.

Bad luck, Safest: RETURN IT, for refund, if it's a bad design the next one will hum.

If they can convince you they can fix it, or a replacement will not hum, you have to decide, make them pay for return shipping if you can.

good luck.

I had a slight hum from my speakers (not transformers) when running a LM805ia SET amp that was effectively eliminated using an Equi=Core balanced transformer. The amp has hum pots so some hum is expected. However, with proper management it can be reduced to nearly null.

If the manufacturer does not offer to eliminate the issue - and it IS an issue - or if they refuse to replace the defective amp, then return it immediately. You should hear no hum from the transformers of a new amp. I have (reluctantly) discarded products that over time have developed a pervasive and musical enjoyment-destructive hum in the transformers. As I understand it this is not something you can fix other than replace with brand new transformers. If you hear it now with brand new transformers, imagine what it will be like as those transformers age.

I have a KT150 amp. There is a very faint hum from the amp on start-up, that goes away once it's fully on and warmed up for a few minutes (it has a 2 stage power up process). Otherwise it is really quiet. 

I sometimes hear a very faint hum/clicking sound from one speaker but I have to get very close to hear it and I know that is from the modem.