Like @johnss said, "add some subs."
I suggest looking at the AudioKinesis Swarm Subwoofer System, which is not expensive for what you get. It is much easier to find speakers that sound great from about 50Hz and up but getting those lower octaves to sound right from your main speakers can really cost some money. There are benefits to multiple subs that you just can't get by using only your two main speakers. If you decide to upgrade your main speakers later, the Swarm will likely keep pace and do a great job filling in the lower octaves.