Welcome!
I built a dedicated room 16 years ago. Suggest starting with the "right" shape (avoid small/squarish spaces, mine is 8ft x 13ft x 21ft - Fibonacci ratios). Bigger is better if you can maintain a decent shape. Insulate (I used insulated staggered stud interior partitions, insulated fiberglass exterior door with weather seal, and insulated/lined flexible ductwork). My downfall was use of recessed can light fixtures, even with insulation above sound transmits. While I was at it installed 3 cryo'd 20 amp hospital grade duplex receptacles, each on a dedicated 12 gauge/20 amp circuit, and grounded together, independent from the rest of the house.
Recommend reading Floyd Toole's "Sound Reproduction" (either edition) to learn how loudspeakers behave in-room and prepare to add multiple subwoofers.