I don't have any problems using 8 inch woofers for my subwoofer. Using a 15 inch or 18 inch woofer takes a lot of watts to begin on producing sound. The room size changes how the subwoofer works. Phase can be off from using 15 inch or 18 inch in a small or big room. Also they tend to over power the audio spectrum if the user is greedy on bass. Using a corner in a room can make a small subwoofer sound big.