>Grounding pin-5 (gnd) on the RS232 plug - disappears
>Grounding the Empeg chassis (much less so against the top cover) while in the sled - disappears
>Grounding the AC RCA outters - disappears

It seems you've gone to a lot of trouble to diagnose it. From what you've said in the above-quoted lines that you've already found the solution. Ground the player chassis or the RCA shrouds and you're done? Or am I missing someting?

One thing you didn't say: Whether you own a Mk1 or a Mk2 player. If you own a Mk1 player, this is actually expected, there were even a set of RCA ground adapters just for this purpose included with the Mk1 player.