The stepups do not have to be that far away. Depends on shielding, etc. Inches make a difference. The main problem was the narrow chassis, forcing the routing too close to the power tranny. A wider chassis would be ok, or routing the inputs into the front side. Basically, don't run the low impedance signals right next to the power tranny. It's a magnetic induction problem, which is why it doesn't show up with 47k loading.
Think of it this way, 6" or closer to the tranny is bad. 12" is great.
jh