To make sure that the hum issue is not being caused by some incompatibility between your source or interconnect cables and the suspect amplifier, please do the following:
Turn the amp off, disconnect the signal input cable at the amplifier, leaving the amp connected to the speaker. Turn the amp back on. Does the hum still remain? If it does, it definately is a problem with the amplifier. If the hum now goes away, the problem is with something ahead of the amplifier.
Does your tube amplfier have a bias setting method? If so check that to see if it will bias set normally. If it does not set properly, you likely have a defective output tube.
One further thought, does the amp have a three wire AC power plug? You may be generating a ground loop betwen that amp and the other components.
Let me know the resuls of these tests. Also, what kind and how many output tubes are used?
Regards,
Frank Van Alstine