Welcome!
As part of our new house build (15 years ago now) I included my basement study (audio in the front, office in the back, HT in a separate room). The room follows Fibonacci ratios (8ft x 13ft x 21ft) and is reasonably well insulated (didn't employ any extreme measures beyond staggered stud walls). Have 3 subwoofers and 10 GIK absorption panels plus <500 Hz Dirac Live DSP. BTW Earl Geddes, a recognized acoustician, has a very similar audio/HT room.
Anyway room size and shape are paramount. Suggest reading Floyd Toole's "Sound Reproduction" 3rd edition to learn how speakers interact with rooms. Bigger the better, but shape is also important (avoid simple height/width/length ratios) to minimize standing bass waves. My mistake was installing recessed can light fixtures (not sound proof even with insulation above), should have used wall sconces. Keep ceiling as high as possible (typical limiting factor in basements). I used lined/insulated flexible ductwork and an insulated fiberglass exterior door. Run dedicated electrical circuits while you're at it.
