Let me give you a very simplified (yet easy to understand) example why some degree of sound treatment is key to improve the listening experience.
Let's assume your listening (ear) position is exactly 1 meter from your speaker. Let's also assume that the same sound travels 3 meters total from speaker to side wall (reflection) and back to your ear. So the difference between direct sound and reflected sound off the wall is exactly 2 meters.
This situation leads to the following effect:
- The reflection of a 1kHz sine wave will hit your ear (6 wavelengths) later than the direct sound ... but "in phase". So what you hear is direct sound PLUS reflected sound.
- The reflection of a 500Hz sound will hit your ear (3 wavelengths) later than the direct sound ... but also "in phase". So what you hear is direct sound PLUS reflected sound.
- Problem is with 750Hz. Obviously the reflection will also hit your ear later than direct sound. But this time 4.5 wavelengths late. So it is 180° out of phase. This leads to the effect of hearing direct sound MINUS reflected sound.
Now if you feel like you need to boost 750Hz, you need to keep in mind that by boosting 750Hz (e.g. via EQ) you also boost the reflection, not only the direct sound. Bottomline: EQ doesn't help at all to fix your issues.
The example above is VERY simplified, but it should help you understand how important it is to manage especially first reflections (which carry the most energy). Best way to tackle these (at low cost) is acoustic foam panels in the first reflection points.
Talking about acoustic foam:
Thickness of the acoustic foam matters. The thicker the foam is, the lower the frequencies it can absorb properly. For example 3cm foam will only handle high frequencies down to 1kHz. If you want to make sure you can handle lower frequencies as well, 10cm foam will help down to roughly 250-300Hz. In my opinion (and experience) 10cm of foam in the first reflection points already helps massively. You will instantly hear the difference and the improvement.