Remove those clip leads and clip them between terminals 5 and 10 on each output transformer (where the teflon wires connect).
If the original test reduced the level of hum, that is indicative of noise being picked up in the wiring going from the RCA jacks to the selector, attenuators, and tube sockets. If the noise is not dependent on the position of the attenuator (which you indicated before), then I would be suspicious of layout issues.
The Mainline is exceptionally sensitive to layout, and any deviations present the possibility of problems, so this may be a rather difficult issue to track down.
