If you're comfortable with lots of tweaking, that's good, because that's what's in store for you. LF characteristics can cause multiple subs to cancel each other out and reduce the bass levels instead of increase them, based upon placement and room shape [you mention that room is a weakness, so this may be a problem]. On top of that, LF being non-directional sonically, you'd benefit more from spending the money on a single, powerful sub than multiple lower-power subs [and who doesn't want a giant sub? ].

If your VTF-3 isn't cutting it in your room, have a look at this one , it's kind of big but I'm told it's a gutshaker. Other Outlaws in the forum also mention some good experiences with subs, you may want to search for some posts.