[morzh]

Tim

Well...a chassis is metal. Magnetic or not, it does not matter.
A transformer has some hum to it. It is natural. There are eddy currents, 120Hz vibrations etc, and so any power transformer will have some mechanical vibrations to it. As well as some leaking magnetic field, which bulges out just a tad out of the core.
The vibrations will make the chassis vibrate. Also the leaking field will create the eddies in the chassis (because it is conductive), and the latter will interact with that field by additionally vibrating.
Some of it (directly coupled mechanically transmitted noise) could be mitigated by what you did. Or grommets. Some cannot. With some the bell (encasing, as we see in many Hi-Fi amps) will help.
Then the chassis (metal, vibrating) will create some capacitive microphonics with something else.

I am not sure I understood this: are you saying that you pulled all the output tubes, and the speaker emitted the hum?
If yes, was the speaker connected to the output transformer, and was the latter powered (through primary centertap, I guess)?