I'd only use the high-pass filter in a plate amp as a last resort. I think it's much better to use the natural rolloff of the main speakers and adjust the low-pass freq on the plate amp accordingly to integrate. My Theatre-3's go pretty low by themselves and I'd assume the 4's go even lower. You'd have to bring the low-pass freq on the plate amp down to the 40Hz range for good integration.

Gar.