I could be wrong, but aren't you using the line out from your Onk to your sub? If that's the case the Onk wouldn't be sending a signal to the mains any lower than you had the crossover setting on the Onk set to, no?

If you had the sub and speakers run in series I can see how a simply crank of the crossover knob on your sub would do the trick, but I didn't think you had it set up that way.

I could be COMPLETELY wrong on all counts above so forgive me if I am.