[Arran]

With regards to how the wiring is routed under a chassis, yes it can make a difference. I had a Sparton AC/DC set that had a hum that would not go away, and after ruling out all of the usual suspects I noticed that some of the filament circuit wiring was routed parallel to the AVC wiring, so I rerouted the length of AVC wiring that was next to the heater wiring and solved the problem. It isn't often that the factory would make a mistake like this but in this case they did, as a rule anything with AC running through it is usually routed as close to the chassis as possible and as far away from the AVC or audio circuit wiring as possible as not to induce hum.
Regards
Arran