> I do realize that the Avant has in Sound settings an option to enable/disable loudness
This will “only” increase the levels of the low and high frequency bands at lower volume settings, as is described in the Beovision Technical Sound Guide.
> as well as clipping protection
This will “only” prevent the analog and wireless power link levels from clipping due to a combination of high signal levels, increases in the tone controls, and playing at maximum volume.
It’s difficult for me to diagnose whether or not there is actually a technical problem, or whether this thread is a discussion of a mismatch between your preferences / expectations and the sound design & capabilities of the loudspeakers. Changing the source will not increase the maximum possible amount of bass that the Beolab 20s can produce – but it’s possible that the settings in your current source (which I assume is a Beovision Avant, based on your comment) are reducing the bass level. However, if this is true, then the Avant is configured to reduce the bass at all volume settings.
In order to make sure that you’re not dealing with incorrect settings in the Avant, create a new Speaker Group with only two loudspeakers (the BL20s) set to Left Front & Right Front – and then choose the Music Sound Mode (and reset it to the default settings).
Also, check your global Bass and Treble settings on the Beovision Avant to make sure those are set to the default values.
From there, you can tweak the settings to your preferences. However, if this does not give you the desired amount of bass at extreme volume settings, it’s because you’re hitting the physical limits of the loudspeakers.
Of course, the placement of the loudspeakers in your room will result in changes in the overall spectrum – but there’s no way to diagnose this or make any recommendations remotely. Generally, the old rule of putting loudspeakers in corners will give you more bass – but it’s typically a very “boomy” and uncontrolled bass due to the coupling to room modes.
PS. I’ve (hopefully) disabled the comments on my website – but I might have missed a page or two here or there by accident.