Sad to see so many who have been here long enough to tell the OP exactly what is going on, and yet they don't. I don't get it. What explains this reluctance to learn?
mizike, this is very common. Bass frequencies bounce around the room, canceling and reinforcing at different points depending on the frequency, room dimensions, and speaker locations. All you can do is move them around trying to find locations that are less lumpy. Symmetrical locations are worst, because they'll double up the cancellation and reinforcement. As you noticed there's bass lower than 40, just none at 40. Move them around you will probably be able to get bass at 40, but then none at 30.
The solution is more subs. This is what everyone here knows but shamefully is keeping from you. Do a search for Distributed Bass Array, DBA, or Swarm subwoofer system. That is the answer. None of the other stuff they're talking about is anywhere near as good. And they know it.
2+2=4
