[morzh]

Basically you see wherever things are grounded using lugs riveted to the chassis (if there are such) and make sure the resistance to the chassis is as low as possible (a typical Fluke DMM will show about 0.2 Ohm). Sometimes oxidation develops between the rivet and chassis and so the grounding suffers.
Long leads of filter caps might matter.
Power cord split inside the chassis and creating a large loop might matter.
Even poor connection between some tubes' pins and the respective sockets might matter.
Heaters hookup to the transformer winding if done improperly might matter. The proper way being, they run in parallel, the transformer leads coming in at the same tube's heater (closest to the transformer) and then proceeding in parallel 'till the last tube.
Sometimes I see the transformer having one lead going to one tube (say first in line) and then the other lead going to the last one. This creates a large loop and the current being sizeable then creates lots of EMI.