Reflections

After the bass treatment, the next important thing to get a clear sound is to get rid of acoustic reflections from walls, ceiling, floor and table.

To find reflections, there is a nice graph in REW, which shows all reflections over time in spikes.
You should measure each speaker solo for the ETC diagram.

As you can see at time=0 this is the main signal, the few crazy spikes to the right of it are table reflections, and everyting else after this are room reflections.

The rule says every spike over -20dB (blue line) is very bad and must be treated, every spike over -25db should be treated.

The best thing to approach reflections are acoustic panels. Out there are so many providers of acoustic panels, so it's just a matter of price and taste which ones you like to pick. Most important thing is frequency response, so you also get some of the bass absorbed.

I decided to build my panels myself. I bought some Basotect plates from a german discounter, built a small wooden frame and covered them in cloth again.