As this is allegedly a settled topic I will wade in only briefly:
- audio truism #1: the room is half your system.
- when my wife and I renovated our 1865 4.5 story brick townhouse based on a Calvert Vaux #5 design from his Villas and Cottages, I turned the 390 sq ft .5 story gable-ceilinged (no right angles) attic into my dedicated listening room. I did not cover the 5”-13” of rock wool insulation in the walls and ceilings with sheet rock. I instead covered it with fire resistant burlap. So: no acoustic panels: the entire ceiling and wall is an acoustic panel. Moroccan rugs cover the floor. The room is semi-anechoic: people immediately hear the difference in the sound quality of the room as they climb the stairs and enter the room.
See
theaudioatticvinylsundays.com