Not sure why more people aren't telling you to add a subwoofer. Before upgrading anything at least get the full range of music in your setup. There is a ton of perceived benefit in having a subwoofer added to your system to give you better low end response. You cross over your mains at 80hz and it removes a lot of the load from your existing amp and speakers. You can also place your subwoofer somewhere else in the room to optimize for low end performance. Don't spend your money on anything else until you have a subwoofer and are hearing all the musical information.
If folks don't believe me just look and Amirs reviews with the speaker preference scores. They all jump up 1+ points when adding a subwoofer. Because smooth low end extension has a large impact on listener preference.