Isolating the secondary merely means disconnecting everything on the secondary (load) side of the transformer. Doing this will confirm that the humming is caused internally in the transformer and not as a result of a too high load. Replacing the transformer is the only cure. Having siad that, the humming transformer will continue to work technially with no problems for many years on, but of course the humming can be annoying. Martin

Thank you Martin; is replacing with used old stock the way to go or is there a modern replacement?

I am not aware of any modern replacements. You could ask at Danish Sound Parts. Martin

Good news: the hum after a bit of use is now barely audible helped also by isolation feet. The amp has stood around unused for years maybe it needed some heat!