I think by far the most cost effective improvement I made to my listening room, aka my living room in my apartment, was my sub. I used to have a JBL E250P and I upgraded to a used SVS CS 25-31+. I ended up selling my JBL quite ironically to the guy that lives above me (he wasn't too fond of it, while in use at least lol) and after everything was said and done I only paid 250 for the sub and the amp that came with it shipped. The sub is phenominal and sounds soooooo much better than my old one and really complements my fronts a lot better. Better extension better everything obviously. Best and most cost effective upgrade for me to date.