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 subwoofers have large rolled rubber surrounds and larger 2" voice coils 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.  Best of all they are affordable.