Joe,
If you want to use the main outs....then Evangelicals way will work, I just tried it.

Out of the L&R mains from the Tyros into the little Y adapter that came with the 621's> the green ended cable that came with the 621> the sub. The 621 satellites get plugged into the 621 subwoofer too.

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jam on,
Terry
http://imjazzed.homestead.com/Index.html