Supercube SC 4000 worth $250 to upgrade from Yamaha YST-SW012?

I got a cheap 5.1 set up from Craigslist a few years ago with some tiny old plastic Sony speakers that probably came from a surround dvd system, and a Yamaha YST-SW012 subwoofer.

Looking to upgrade now, and I found some used Definitive Technology ProMonitor100 mkii bookshelves w/stands and a center on Craigslist. The seller also has a couple of Supercube subs, the SC 2000 and SC 4000. I was just going to pickup the bookshelves and center for $160 but now I'm wondering if it's worth a couple hundred more to replace the Yamaha sub.

Thoughts?