Same thing happened to me in my Klipschorn room. Changed out some nice cloth (sound absorbing) furniture for some all leather furniture.
The K-horns became very lively & "reflective" in the room. When loaded with people (or some sound panels (Echobusters)), the legendary sound was back!
Live & learn! Everything, I repeat EVERYTHING, makes a difference... some extremely subtle, some not so...
My $0.02,
Pete