Sound quality vs. volume

Looking for a bit of expertise here:

I recently made a few changes to my setup and while overall pleased with the results, I’m on the quest for better.  I’m hoping you all can help me diagnose an issue I’m hearing.

When listening to music at lower volume levels - say less than 1/2 total volume, the clarity, imaging and dynamics come across far more coherent and “in focus”.  To use an often over-coined phrase “It’s like I’m there in the room”.  As I start to push the volume up a bit, closer to live-performance levels, the sound becomes increasingly “mushy”.  I know, a highly technical term, but the best way to describe what I am hearing.  The bottom-end loosens up - getting a bit boomy, the crispness of the mid-range and highs fade and the imaging falls out of focus.  These are all incremental with volume until I get to the point where it’s just unbearable.   

I’m no expert by any means but feel it might be room acoustics.  I already know I have a less than ideal setup with a nearly square room (21x20ft) with 60% of the surface covered with clear birch wood paneling. Some things we can’t change (easily).  I do not have any acoustic treatment, just lots of soft furniture.  What I find interesting is that my old setup (Magnepan 1.6) didn’t suffer to such a degree.  Maybe with the new setup there is more to loose?  A mystery.  

For a bit more context:  
Speakers:  Dynaudio Contour 60
Streamer: SoTM sms-200 Ultra
Amplifier: Peachtree Nova500

Within the 20x21ft. room, my speakers are 4ft. from the wall, I am seated 13ft. from the front wall (a bit back from room center). Speakers are 9ft. apart.

Any thoughts?  

@wanderingmoo. In my opinion the compression that happens at higher volumes is related to electronics and room . I can listen at 105 dB in my listening chair after changing electronics ( monoblocks, preamp, dac) from parasound to ayre and cables from kimber select to cardas clear without compression. Speakers and room acoustics are the same . The sound opened up at louder volumes . The performances sound real and there . 
Room acoustics and smearing I had talked to acoustic sound company , they sent me an audio file via email and we did the “Matt test” to identify reverb and echo in my room in my listening chair . They then made their specific recommendations. Nothing random, nothing textbook . But based on data collected from my room. You can contact them and they can help you. They work with the decor of the room and waf also 😊. Good luck . 
Let me say this about that.

Once anyone gets a very satisfying balanced system and speaker/room setup, most often you will find you very rarely listen as loudly as you used to. You are busy being immersed, involved, hearing things previously undiscovered.

Once a level of involvement is achieved, it is time to move to superior content, broaden your interests/experiences, move into phono if not there, move into Reel to Reel if not there, ... all of which enrich the experience far beyond tweaks.
It's the amp that causes 'mushiness' at higher volume levels. Upgrade your amp.Some big/heavy "high current" mono blocks.
The last three posts are correct in their own way except newtoncr 105dB is pushing the danger point for your ears. Tweek the bass and the treble up just 1-2 dB and 95 dB will sound just as good. Elliot you can have a marvelous sounding system that will not push comfortably beyond 85 db. But id you have a system that will do 95 db comfortably that is heaven for some music not all. A string quartet at that volume would sound rather ridiculous. jkbtn right on.
Unfortunately, distortion sets in way before high volumes are reached. There is distortion even at moderate levels. But the higher the volume the higher the distortion. You’re just used to it, that’s all since it’s always been there. You’re unaware of the distortion until it’s removed. It doesn’t have anything to do with amplifier distortion. It is more mysterious. Much more mysterious. it’s not really a secret because no one knows about it. 😲