I've had a few plugs work their way out of my older Marantz unit as well. Locking plugs do work and I now use them on the amp side of my gear behind the rack because I spent a lot of time troubleshooting a dead channel only to find it was a banana plug that had slipped out of the back. I don't use them on the speaker side because I'd rather have them pull out or even break than knock the speaker over.
IMO, it is the plugs themselves that lose spring and that's why they come loose. You can use a small screwdriver or knife to "retention" the spring part. That's the reason I switched from standard plugs to BFA style, as they tend to retain their spring much longer than standard plugs. Those that use set screws to retain the wire, with a sleeve that keeps those screws from backing out also keeps them on the wires.