B&K measured the room/speaker response which listeners actually hear. Sonarworks repeated the research and the curve remains essentially the same 50 years later. I believe to mix for these people we need to monitor using the same aggregated tonality. A Zone Without Reflections will lack the Bass Boom of normal rooms. Also it will have extraordinary clarity due to the lack of HF destructive early reflections. Dead Tight Too Bright, tonally, but with great clarity for listening into the Mix. Mixes done in such a tonality will inevitably be dull. You have the evidence. Removing treatment would reinstate room tone, which would be self defeating. Keep the anechoic benefits, but apply the Bruel and Kjaer curve using Monitor Eq or the controls on your speakers, or both. The Bass boost part of B&K can often be achieved by moving the speakers to almost touching the Front Wall.