Acoustics, using placement as EQ

With help I have had a recent journey with this issue. The only subwoofer placement strategy that helped was lifting the subs half way off the floor. In reality the majority of the problem came from my speakers and no x-y placement was going to cure it. A crossover eliminated it. Have you measured your speakers with and without the sub?

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.

So what is the actual size of the room?

Where are you measuring at?

Turn the subs XO to 60 <. It would be nice if you could get at least a 12 db 2nd order XO and 24db 4th order will take that bump right out of there with a lower XO point on the sub.

50 and 80hz are ceiling height issues. That I've found. 8 foot and 10 foot ceilings.

You control all frequencies with the same ideas.
Limit or increase (EQ)
Diffuse the frequency
Absorb the frequency
Remove the reflective surfaces. Open, back wall, or open in a warehouse environment. (A panel and stage curtain set up) One of my favorites.

A real easy way to tame and correct is just add more subs. Forward firing no downward firing unless it's an enclosed slot.

Decouple. Plain and simple, not part way, ALL the way. Airbags, springs, pods, take your pick. 

The type of sub you use. A Servo system, vs DBA or Swarm, vs Sub columns and any combo. Servo sub columns... Column swarm or DBA.

OP so you know, you don't get bass BS with a servo system and it's a turntables delight for any rumble issues..

Regards

Sometimes shifting your listening position a bit will help with your 71hz node. Try moving your chair a foot +/- forward and or backward and see what happens.