But there's one other quetion on my mind: if I power the empeg directly from the battery, will I need to use a relay for the switched power lead?

No, you will still use the ignition-switched wire for the switched power lead. Only the constant-power wire should come straight from the battery. (You do need to put a fuse block on anything coming off the battery, remember. Mine is a weatherproof 20a fuse in the engine compartment, only 6 inches down the line from the battery.)

Just for diagnostic purposes, you can try connecting both leads to the constant battery power and see if that gets rid of your noise. Although I don't expect it would change anything.

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Tony Fabris