Completed my two channel listening room a bit over a year ago. The quality of silence is surprising; guests remarked upon it as soon as they entered, in pre-Covid days.
Dimensions. Snake oil abounds. Fortunately, the real science has been done, at the University of Salford (UK), in their School of Acoustics. The famous Cox teaches there. They ran 100,000's of simulations to arrive at optimal ratios - they found that MOST rectangular dimensions are bad, a quarter are OK, and a few percent are good.
Stereophile ran an article about two years ago on construction techniques. They stressed rigidity and glue. I used an elastomeric glue that never quite dries, and it's elastomeric counterpart caulking, called Build Secure and M1 respectively (from Chemlink).
Walls and ceiling used 6 layer drywall including an embedded layer of sheet steel, called Quietrock 545. The manufacturer is knowledgeable and very accommodating. Construction techniques are also discussed on their website.
Isolation transformers tend to hum when they are doing their job. Best to site them outside the listening room. Don't forget air lines - you just may end up with an air bearing TT or tonearm, or both.
Good luck!!!
