grmeade...
Hold on for a second here...I wouldn't really advise you to run out and buy the cheapest, first sub you find. A small 8" sub for $100 probably isn't going to be on the same level as your Titans in terms of performance (of course I haven't heard this unit, but Yamaha isn't known for their speakers these days). I'm worried that this sub might be what we call a "one-note sub". Lots of boom, but the boom always sounds the same. And for $100 at 8", I doubt that one note is "sub" anything.

You'd be much better off IMO to look at the used market - ebay, audiogon, etc...Or take your time to save up some money for a better one. I would shoot for a 10" sub as a minimum..but that's just me.

You should be able to find some ideas for a sub here, or post a new thread.

Good luck!