Well, find the wire(s) (inside the empeg) going to the mic connector. Disconnect it/them from what it/they're currently connected to, and connect it/them to the sub out board instead. Then get an 3.5mm mono (?) jack to RCA adapter. Plug adapter into mic connector, RCA wire into the resulting RCA jack. Other end of RCA wire to sub amp.

Couldn't be easier

Of course, getting a sub out board before the meet (forgot when it's planned) might not be realistic....

/Michael
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/Michael