Thanks for the eBay listing, that definitely helps. Taking a look at the inputs, I think you can make it work. The subwoofer is looking for what's called a line/low level input, which is what would be coming out of the "Sub Out" on the BT30D. You would just want an adatper cable like this one to go from the RCA on the BT30D to the 3.5mm input on the sub.

The majority of standalone self powered subwoofers you can find on the market will have RCA line level inputs or a LFE input, which will work just fine with the BT30D.

If you're looking for a half decent affordable sub for desktop use, I'd take a look at used Definitive Technology subs. Most of them have lots of input options, and the 8" Powerfield Subwoofer in particular goes for $80-120 on Craigslist/FB Marketplace. I'm personally considering getting one for my desktop setup.