First, it is main speakers location and listening position as much as sub location. Bring speakers and chair out into the room more. Typically each roughly 1/3 of the way is pretty smooth. Then after this the problem is one sub always does this. You can move it around, all that will do is shift a few dB or Hz here or there. What you need is more subs. With 4 each one puts out 1/4 the volume. This lowers the peak and at the same time raises up a lot of lower frequencies. The result is a lot smoother overall response.
All rooms have this by the way. All rooms are small relative to bass waves 50 to 60 feet long. One room your peak might be 60, another 80, in yours 71. Big whoop. Same deal. Same solution. Works every time.