The most dramatic improvement you will ever make to any audio system is by adding a subwoofer.  D1 series 2 subwoofers make it easy.  These new subwoofers have large roll rubber surrounds and larger motors for better power handling than competing models.   They are available in either dual 2 ohm or 4 ohm versions.  They work equally well in small sealed or medium vented enclosures.  They sound great.  And best of all they are affordable.