Though I do not disagree with VMPS' advice on the VTF3, sub, the
VTF2 is also a good sub. It is smaller, too.
What is the size of your room? Please include total open volume of the room, not just your "area." (Unlike speakers, where distance-from is a key factor, Subs will see the whole open room volume.
If you don't know, or can't math it out, cool, but anything to help us guide you!
)
Regardless, getting a decent subwoofer is one of the best upgrades you can do for your system. Properly set up, it will not overpower your system. It will add clarity to your highs and lows both. Good quality Bass, clean and distortion free, is a very wonderful thing. I suspect the VTF2 will be a significant step above that MK, and just about anything else under $500.