Room treatments

 I guess I would have to give the most credit here to Mr. Michael Green of Room Tunes fame for popularizing room acoustic panels for the masses. These were mentioned in the same breath as Monster Cable back in the mid '80s. I still have a pair of Room Tunes (above) and Corner Tunes in my studio. He is still in business today :

                                                  http://www.michaelgreenaudio.com/

                                        The competition  followed, ASC with their Tube traps,

     Aurulex and numerous other companies with their sculpted foam sheets and corner pieces, and Echo Busters which are very similar to Michael's designs. There are literally hundreds of these companies making this sort of thing today, mainly for recording studios, but they can cure significant problems in your room (echos,delays,boomy bass, etc). Try before you buy applies here, and many tend to overdo things by using them to excess.