Abaca Audio 4 Inch High-End Shielded Woofer 4 ohm: perfect for replacing/upgrading or building bookshelf, surround, sound bar speakers, or other diy speaker projects.
Building A DIY Sealed Subwoofer | Audioholics
ONE Cambridge Soundworks MC300 4" woofer - Great for DIY! HIGH QUALITY! | eBay
The Best High-Performance Subwoofer | Reviews by Wirecutter
Research and DIY audio projects - AudioJudgement.com
How to Design Your Own DIY Subwoofer - TurboFuture