Room Acoustics

Excellent willemj! The DSPeaker Anti-Mode 2.0 Dual Core comes with a measuring mic, and has a screen that displays the frequency response of the room, room modes often creating huge peaks and nulls in that response, in both the frequency and time domains. The Anti-Mode then creates a corrective signal to counteract the peaks, but nulls are often uncorrectable---you must move the speakers to deal with them.

There are room mode calculators on the web, into which you insert your room's dimensions. The calculator will show you the physical locations of the room's modes/standing waves, and their frequencies. Bass traps can be placed in those mode locations, and you want to avoid putting your speakers at those locations---they energize the modes.

Higher-frequency reflections are a completely different matter, and can be dealt with via diffusion and/or absorption, as suggested above.

Actually, his room only has two areas of problem nodes in the bass area:

https://amcoustics.com/tools/amroc?l=13&w=12.5&h=6.5&ft=true&r60=0.6

Two lower nodes at 43Hz and 45Hz, which are going to be very hard to deal with unless you use a tuned membrane (like a GIK Scopus T40 -- or have them custom make a T44)

Two upper bass nodes at 86Hz and 89Hz. The GIK Monster bass traps or even the GIK 244 bass traps are going to be very effective at treating these, as long as you stick them in the corners.

The first reflection points can be dealt with from DIY Owens 703 panels or the GIK Acoustics Spot Panels.