Why does pulling out main speakers from wall improve sound?

SBIR - Speaker boundary interference response.

As noted, perhaps not fully correct, but as the frequency drops for a dynamic speaker, the radiation become omni-directional. The wave reflecting off the front wall is reflected, with not a lot of attenuation, and then cancels the front wave of the speaker. The critical distance is 1/2 wavelength as that will be completely out of phase.

Ideally you want to be far enough from the front wall that the round trip distance is > 1/2 the wavelength of a fairly low frequency. Put another way, the distance is > 1/4 the longest wavelength. 3.5ft, and you just created a suck-out at 80Hz. Even at 5 feet, you are still hurting important bass frequencies, and this is not stuff easily corrected if at all with DSP.

Counter-intuitive, but the solution is to put the speakers closer to the front wall, and then treat the walls with absorption. Not those those trinkets I see in audiophile pictures that cover a small portion of the wall. You need to cover a good portion of the space behind the speaker, and the absorption need to work at a low enough frequency to prevent the SBIR cancellation. You will get boundary reinforcement by being close to the front wall, but this can be corrected with DSP.

Great discussion. Much good info. Varies much between speakers but the answers that addressed “baffle step” ring with me (see what I wrote there). As a Wharfedale Linton Heritage speaker owner, I’m a fan of the current wave of throwback big boxes…with the right distance from the back wall and a little toe-in they seem like best of both worlds between crisp imaging and rich tone.

It also lowers the noise floor. Like each and every time your audiophile friends come over to listen to music and say: "Hey, you really ought to move those speakers out away from the wall".

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@danager :  My issue is the bass sounds so much better when I stand up. 

I had that experience before too.  What I found is that my speakers were too far from the front wall.  It took me a couple of days of effort positioning the speakers in 1/2" increments to finally get them dialed in.  That includes the distance from the side walls too.  The crazy thing is, once I got them dialed in, the bass was nearly uniform all around the room- not just in the listening chair.  A good CD for evaluating the bass is Stanley Clarke's East River Drive.  The first track has a double tap of the bass drum.  When I got everything right that drum sounded good, not muddied to almost inaudible- except when standing up.