the qsc probably has a high pass up around 35hz or so, so not really good for theater where 30hz is kind of the king daddy.
since you can build a box and the 18" driver isn't that much more expensive, why not just build a medium-large ported enclosure for the 18" and tune it to something around 20hz. that would be an upgrade from what you have now. then, when you get the bug again, you can just build another one. :-)
there are some cheap eq options such as the berry mic2200, mini-dsp, and so on...
the berry amps are spec'd at peak, so keep that in mind when shopping around.
if your nor familiar with winisd, it is free and can model subs and enclosures if that is something that you would be interested in doing. otherwise, ping a couple people and somebody can model up what your thinking about.