You can't have too many bass traps...

The big/thick bass traps can help some, but they still do not absorb much down low (such as 50-60hz).  Also, since they are broadband, they will tend to suck the life out of the room if too many are implemented.  I have actually found that Owens 703 FRK panels work very well down to about 85 hz.  And they are not huge (only 2 " thick).  Very light.   The have a metal foil that covers the front surface and acts like a tuned-membrane bass trap, but still reflects the upper mids/highs.  The foil resonates with the bass frequencies and the fiberglass behind it will absorb that resonate energy.  You just have to be careful not to put too many in your room because it can start getting to bright/harsh.  They are excellent for handling the upper bass and midbass.

Tuned membrane bass traps or limp mass bass traps are the only ones that I have found effective for lower frequencies.  I tried a variation of helmholtz resonators and it was a pain and didn't do anything.  I didn't do the 6" / 15foot design, though.

You can build your own tuned membrane (but don't use dense panels for the inside):
https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html

Or you can get GIK's version, which are the Scopus bass traps.

I have also tried the GIK bass traps with the FlexRange limiter.  They are okay, but don't work as well as the 703 FRK panels.  The FlexRange limiter is really just a 1/8" piece of cheap wood that is loosely held in front of the acoustic material.

Well, that's why in my original post I said one needs to define what a bass trap is. Generally speaking, people buy "Bass traps" to tame all that low bass output. Whether they are right or wrong, that's what people do.....

Bass traps will absorb bass frequencies.  The idea is to absorb this energy so that it does not get reflected back into the room.  The biggest problem are when reflected bass hits the generated bass from the speakers and cancels itself out, creating a bass NULL.  This can be heard as a significant drop in bass around certain frequencies.  Bass traps will reduce this reflected energy and reduce the bass NULLS, therefore allowing more bass to be heard.

Here you go, 4 Acoustic Myths:

http://https//audiophilereview.com/room-acoustics/the-top-4-acoustic-myths-that-need-to-be-consigned-to-the-garbage-can-forever.html

The biggest problem are when reflected bass hits the generated bass from the speakers and cancels itself out, creating a bass NULL.

I'm going to disagree.  I've measured 20 dB peaks as well in a modest living room.  That peak prevented the subwoofer level from coming up. Nulls, if narrow enough, may not even be heard, peaks though are ugly to listen to. 

I'm going to disagree. I've measured 20 dB peaks as well in a modest living room. That peak prevented the subwoofer level from coming up. Nulls, if narrow enough, may not even be heard, peaks though are ugly to listen to.

Not disagreeing with you.  peaks can also be a problem.