speakers against wall a good thing.

Lots of people, including professional recording studios such as Abbey Road, use their speakers against, or sometimes even in the wall to mitigate any possible SBIR dips.

The common audiophool notion that "But then all the musicians become squashed flat like a pancake and the music has no depth to it" is mostly a myth however there's a grain of truth to it: by placing a speaker against the back wall you kill half of the room reflections it produces since the rear hemisphere output no longer bounces about the various room surfaces before reaching your ears. Since room reflections create much of the "ambiance", "liveliness", and reverberance" to the sound you have altered how they sound in ways that do convey a sense of "depth" [sometimes too much].


The proximity of a speaker to wall boundaries [floor, ceiling, and walls] also increases output, predominantly in the bass. The more room bouncaries touched the more bass, hence room corner placement on the floor or at the ceiling creates the most bass (again, sometimes too much):

This placement is sometimes referred to as the speaker's "pi".

Learn more here.

https://www.prosoundtraining.com/2011/08/29/how-boundaries-affect-loudspeakers/

What he said. ^^^

There are (at least) a couple of other considerations:

- Rear-ported speakers should be spaced enough to allow the sound from the port to "spread" before hitting a boundary. The rule of thumb is to use the width (and no less than half the width) of the port opening to provide a little margin. For a rear passive radiator, half the diameter of the radiator.

- Magnepans and other dipole speakers, as well as those having a rear-facing driver, should be spaced far enough from the wall behind to get reflections delayed enough and comb filter effects low enough in frequency to be "inaudible". The wave from the rear hits the wall behind, reflects, and travels back toward the listener. It is delayed by the round-trip distance from speaker to wall behind and so arrives out of phase with the direct sound from the front of the panel. At certain frequencies the two waves will cancel, creating nulls (dips) in the response, and add at others. The resultant reponse looks like a comb with the teeth representing nulls and the back the peaks. Getting the fundamental nulls low enough means moving the speakers away from the walls (Magnepan suggest about 3 feet or more IIRC) and/or killing the back wave through absorption or diffusion.

HTH - Don