Jeff, I'm not sure about this. I might be mistaken but I recall that Chris, in this thread, once mentioned that MultEQ does provide a high-pass filter for the sub.
"Anyway my only question here is does the Audyssey MultiEQ XT32 in my Denon 4311 run any kind of subsonic filter?"
And here was his answer:
"It's not really a separate subsonic filter. It measures the roll off of your sub and then stops applying correction below that point.