I'm fairly certain that my old Pioneer didn't use relays to fully mute the sound. Perhaps more expensive Denon receivers don't either.
As near as I can tell, the relays appear to be buried inside the receiver, making it difficult to get to them without major disassembly. If the relays were socketed, that could also explain the sound level changing.
I like your idea about changing the way the mute works. So, I've changed the Mute Level setting in the Volume Control from "Full" to "-40dB". So far that something I could live with, and also I don't have to worry about accidentally hitting the mute button.
Thanks for the suggestion.