You didn't specify whether you now have a VTF-3 MK2 or a VTF-2 MK2. Especially if it's the VTF-3, you might consider keeping it and getting a second sub as similar as possible to it. This would increase the net output available and probably more importantly, if properly placed would help smooth the bass response by minimizing the effect of room modes. The general suggestion from the research of Dr. Toole and others is placement at the mid-points of opposite side walls or front and back wall or in diagonally opposite corners.
For example, the Hsu VTF-1 could be added to the VTF-2 MK2, or the VTF-2 MK4 to the VTF-3 MK2. Your budget would of course also support some excellent single subs, such as the Axiom EP500 I use or the Hsu ULS-15.
