In reply to:

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If you've just got a SPST switch between the mute and 12v lines, there should be no difference between when it's open and if it were just unconnected altogether, which is how I imagine most empeg installs are, including mine, and I don't get random things happening to me.

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Good point

But I note that the player has settings for both active-high and active-low mute lines; only one of these would work with a normally-open switch and no pull-up/pull-down. And the default player behaviour is to ignore the mute line, isn't it? Which means that I wouldn't expect you to get random things happening to you. Well, not as a result of the Mute input, anyway.