your on the right track. the diagonal room set up is probably the best direction for a small 12x12 room. i once had a system in a 12x12 room, i ended up with smaller two way monitors to get the most even bass response. anything with real bass output simply overloaded the room.
also, sit nearfield if your speakers will allow. sitting nearfield helps to remove the from the sonic picture. experiment with placement, its free and your learn while doing so.
just a thought but the turntable in the corner could be troublesome...as in negatively effected by bass energy.
