So, as i posted earlier, floor bounce frequency is dependent on various distances. I have an average sort of family room for my home theater. The center speaker is just under my flat screen and I sit on a leather couch about 10 feet away. Let's plug these distances into the floor bounce calculator and see what we get...

So for my listening situation, would expect a primary null just under 400 Hz. If you sit further away, the null goes up in frequency further.
Anyone else have a much different listening arrangement?