If you can't do it at the receiver, the universal solution is to pull a wire at the speaker connection.
Another technique which I use when dealing with these time domain issues is to tweak the sub delay at the receiver. While not a phase setting in the strict sense delaying the sub much more than measured distance will tighten the sound and also help get the woofers moving together. At a minimum add at least 3 ft or a metre to measured distance delay to account for the DSP in the EP600. Assuming the sub is in front of you, add still more delay regardless of distance if bass notes sound less distinct when the mains are on.
