Bruce,This is an interesting question/problem. With so many items using switch-mode power techniques, the potential sources are legion. It's interesting that you note high stability - I'd not expect that from LED switchers, but do not have any specific knowledge to support the notion.
Thinking about times I've noted interference in the LW range, it was definitely evenings, and almost certainly a local source. My suspicion at the time was a large flat panel TV - these have extensive switching power supplies. As an aside, the monitor in the laptop I use for HiFER detection provides a selection of stable junk harmonics on 22m, which frustrated my plans to try mobile HifER listening with the Belka receiver.
Another definite LW offender was a cheap phone charger, capable of wiping out, with a piercing squeal, reception of the entire LW broadcast band in a nearby domestic radio with a ferrite rod antenna.
It would be instructive to keep a message list going with details of "known offenders" as they are detected.
I'd hoped we might be done with this sort of thing with the demise of fluorescent tubes. As a teenage radio nerd, my listening post was my bedroom, located above the garage of my parents house. How I curse those garage lights...
73 to you,
Ed KO6BLM