I've been curious about using multiple subs for years so in the last year I've been buying cheap subs off ebay. There's a custom install only brand named Episode and I bought two of their 8 inch sealed subs and one sealed 10 inch model. I paid around $150 each for the 8 inchers including shipping. At that price they're great. They're reasonably flat to around 40hz where they drop off pretty quickly. Since it's a custom install only brand it looks like people don't have any idea how to price them so they can be found at great prices somewhat regularly. They look a lot like the PSB sub series and if you look at some pics the amps are clearly made by the same company.
PSB SubSeries 350 Powered subwoofer | eBay
Episode ES-SUB-EVO10-450 Powered Subwoofer - Gloss White - Tested | eBay