I eventually worked out a good set of search terms and found http://empeg.comms.net/php/showthreaded.php?Board=empeg_tech&Number=11510

So I'm about to try (0x00, 0x20, 0x40, 0xFF) as an approximation.
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Toby Speight
030103016 (80GB Mk2a, blue)
030102806 (0GB Mk2a, blue)