So the problem with my Walkabout(or one of them at least) turns out to be the switching contacts on the input jacks. I'm going to change them out to the current arrangement of one jack and an active/passive switch later today (unless it turns out to be easier to sub in Switchcraft 12A jacks, which take at least a couple of decades to develop these kind of problems!). Obviously, this only applies to older units like mine with 2 inputs.