are you talking about a swimming pool? I remember when I built my listening room as a separate building deep into the earth my wife claimed it would have been better if I had decided going for a swimming pool.
Nevertheless when you have the chance to build it up I would go for a good concret basement, some layers of isolation treatment, then a wooden floor of heavy large pieces at least 40 mm thick. On top of that I would work with carpets and isolation treatments again as you are already doing. Don't forget a proper treatment of the walls and the ceiling. This is such crucial. It is not only the floor. The waves have to be vaporated at the side walls and also look for proper reflections of the different frequences coming from the ceiling in front of your listening chair. This is very important for the real good sound you are going for. Good luck, too.
Nevertheless when you have the chance to build it up I would go for a good concret basement, some layers of isolation treatment, then a wooden floor of heavy large pieces at least 40 mm thick. On top of that I would work with carpets and isolation treatments again as you are already doing. Don't forget a proper treatment of the walls and the ceiling. This is such crucial. It is not only the floor. The waves have to be vaporated at the side walls and also look for proper reflections of the different frequences coming from the ceiling in front of your listening chair. This is very important for the real good sound you are going for. Good luck, too.