By the way, the above post is assuming you only want to spend the $700 you made on your KRK sale. But even assuming you have more money to work with, you still won't get a big huge improvement on what you had before, at least not with the near-field monitors. If you have more than $700 to spend, I would still get active speakers with larger woofers for better bass extension like the Behringers, and the best subwoofer you can afford. I would stay away from studio subwoofers, they won't ever be big on deep bass unless you are spending thousands.
For just a bit more spent, I might go for
these JBL monitors over the Behringers, not that the JBL will sound better but the Behringers don't have a great reputation for reliability in their electronics. If you can handle a big subwoofer, check out the 12" subs from Hsu Research, Rythmik, Elemental Designs, SVS, Epik Subwoofers, and Outlaw Audio which are in the six to seven hundred dollar range.