@woots I went back through the menu on my Onkyo. It does have a crossover frequency adjustment for the center channel. I have it set at 60Hz, so I assume everything below that is going to the subwoofer? My Sonus Faber center channel speaker is rated at 65-24,000Hz.
I guess I could set it to full range and use my NHT subwoofer. Set the crossover frequency on the NHT so only signals above 50Hz to 60 Hz go to the center channel speaker. Doesn't seem like that would help much versus letting the low frequencies go to my main subs.