The seat should be 38% of the room length close to the rear wall, the front speakers in a 60° angle. The Subwoofer (You need one (!) filling the octave from 20Hz to 40Hz which your standard speakers cannot play) should be 38% of the room length close to the front wall at the left wall.
The floor should be carpeted, (carpet's better than wood) 50% of the ceiling (in the center of it) are to be covered with omniffusors (
http://www.rpginc.com/products/omniffusor/index.htm), 60% of the rear and right wall should be covered with diffractals (
http://www.rpginc.com/products/diffractal/index.htm) from the floor to 1.8 meters above the floor.
So much for diffusion. 60% of the left and front wall should be covered with 10cm RPG BAD Panels (10cm Rockwool's OK too). The entire rest of the room should be covered with gypsum board.
You will also need bass traps.Think in terms of: what's the largest I can make ‘em and still get in the room rather than what’s the smallest I can get away with. The rear corners are the most important ones. Make corner bass traps as shown in the attached picture above. The basstraps should go all the way up to the ceiling or at least 1.8 meters high. Fill the inside of the box with always 2 inches of Rockwool and 1 inch of bubble wrap. Make the box from 19mm MDF. THE BOX MUST BE SEALED AND 100% AIR TIGHT!! The front of the box should be covered with Absorbtion material (RPG BAD Panels 10cm) The bases WILL travel through the wood. It's a common mistake not to seal the box. Then you can make as many bass traps (Sealed and with an absorbtive front cover) as you can fit into the room. They must be at least 25cm deep.
Depending on how large your room is, the costs should vary between 3000 and 5000 Dollars if you build the diffusers by your own. Show me a better home-suitable room setup than this; goodbye:! I'm off on a 3 months vacation in the carribean.
If someone disagrees with all of this, please report.