I think the dip you mention is very much room-dependent. I don't encounter a significant dip in my room until 110Hz. This is a result of comb filtering due to the distance to my rear wall.
The cross-over filter slopes are usually not specified but I believe that most are either 12dB or 24dB per octave. I've been told that they are 24dB/octave on the Denons. So if I set the cross-over to 80 on my Denon, by 160Hz the response is down by 24dB. Therefore by 90Hz, the response is already down by 3dB or half-power.
