Ideal design for a new music room

fwiw. Don’t try to reinvent the wheel. Rectangular rooms work and I can’t imagine why you say the Golden Ratios are hard to apply- but- you are wrong. Someone suggested 3/8 inch walls- I can’t think of a kind comment about them. Along with the Cardas site look into the construction features of the Rockport Technologies listening room. The best advice you will get is to decide the size room you want and build it solidly and symmetrically. Allow 5 -6 ft behind your speakers and at least 4.5 feet from speakers to side walls. 8 ft behind your head. Plan a vibration isolated area with a door to the side of your speakers for electronics and vibration isolation for your amplifier. Organ music right?  Assuming you’d like your speakers 6-7 ft apart you need 16-17 ft width, about 19 length. 9 ft is contemporary minimum height for a classy space. Get yourself graph paper and draw to scale-precisely. Use 2-3x5/8 drywall on walls. I’d use a base of 5/8 mdf as the first layer behind the speakers if your speakers can drive organ music down to about 30cps. I did these with my 17x27 rm and got positive comment.  Floor shd not flex. Symmetry helps to kiss. Helps resale value and decorating as well :) Enjoy and best wishes.