I have built speakers for over 25 years and this sd215 is versatile and capable when up against drivers around this price,and as good as many pre made speaker set woofers found in mid and upper fi main towers,or sub cubes. They work well sealed, or ported. They can work as a woofer in a three way system,or as a dedicated sub. Why not 5 star? Not my issue ,but there are many who might find them not as aggressive as bang and slap bass fans would go for. That said they have good power output and handling but you would want to build a multiple driver set up (2 or three woofers in each box) for a more aggressive output. They have a surprising quality of sound for the price point ,they can work well for refined low end extension, but with proper box size selection ,and planning they are not weak. They are popular for blown speaker replacement, but if you known what your doing they are a good value for building diy speaker sets ,or subs without busting your wallet.