Bridgman, you rock! Thanks for taking the time to even draw out the room! Some additional details about the room.
1) The 20'x15' dimensions of the room are approximate, based on the floorplan that the builder hands out when visitors look at their community. It's likely that the exact dimensions might be something like 20'3" x 15'7", or whatever. We're still in the process of finalizing the design for our house, so I don't have exact dimensions from the blueprint. Which leads to...
2) The 15' dimension of the room is negotiable. On the other side of the north wall is a 19'x20' gameroom, which my wife is willing to let me "borrow" a couple of feet (along the 19' dimension) from for the media room. Would this help prevent of the problems with certain frequencies?
3) The ceiling is actually about 9' high on the east, west and south walls, and slopes to 10' high a couple of feet into the room along each of these walls. It's not quite a tray ceiling, though I don't know the exact term for it. Will this affect things any?
4) The back (western) 8'x15' of the room will be raised 1' above the front (eastern) 12'x15' of the room. I assume I'll need to have insulation placed in the 1' riser so that it doesn't cause unintended echoing. Or can I somehow manipulate it to some advantage?
5) For the speaker wires, I had to ask specifically for 12 AWG wire, or the builder was going to put in 16 AWG wires. Are there different type of RG6 cables, or is that already as specific as it needs to be?
Thanks for everyone's input so far!
