Huge! Start with treatment of the first reflection points for each speaker. All rooms will benefit from that. Next try panels between the speakers, about one foot from the tweeter point. Then treat the back wall. Do all in a step by step fashion. While many rooms sizes are fairly standard, what and where things are in the room play an important role in the sound you get. Be prepared to do a lot of listening as you make each change. The room is the ultimate component, needs constant tweaking.
