[Chas]

Let me say this about that....

Change the load on any radio/tv power transformer and four things will happen...
1- Increase the current on any filament winding the transformer will heat more, B+ will go down and primary current will go up. Other supplied voltages will go down. Magnetic field around the transformer will increase.
2- Decrease the current on any winding: All voltages will rise on all windings, the primary current will decrease. Magnetic field will decrease.

That said, the transformer is a finite supply of energy. Can't get something for nothing.

Chassis and power transformer mechanical hum can be an overall design issue but overload can be a factor.

Magnetic field can influence nearby chokes and audio transformer when that would not normally be an issue.

Strive to keep modifications within limits of design as much as possible. A plug in solution is, well too easy...

Type 80's are not rare, just be sure both diode elements are relatively equal in emission as a weak side will yield an un resolvable background hum from this out of balance condition.

YMMV

Chas