For $500, if you have the skill and the tools, you'd get the most 'bang (or should I say boom) for your buck' with a DIY sub.
You could also buy a separate plate amp from Parts Express as Jonnythan noted (
here's a few to choose from) and replace the C-V amp. These almost always have a built-in crossover and high level inputs to use the crossover settings of the amp. If this were the chosen route, buy the strongest amp the C-V can handle. I believe there's a decent 300w B.A.S.H plate amp available for just about $150 that would suit the CV perfect, assuming the driver wasn't damaged at any point and you liked the performance of it.
Having said that, for ~ $500, you can find a pretty solid sub for HT as well as music use. How big is the room you'll be using it in? What's the majority of your listening material? Personally, I would suggest watching
SVSound's website for B-stocks like a hawk and then pounce on a PB-10 or PCi 25-31 as soon as it's available. Both B-stocks likely can be found for under $500... -TD