15-inch speakers are stage singing speakers, good midrange, bass is poor, enjoy the music can be complemented with a subwoofer subwoofer.
Subwoofer power and two speakers power is almost on it. 18-inch subwoofer power is relatively large, bass frequency is good.
Home audio, 15-inch unit has been considered very large, the low frequency should be able to meet the requirements, there is no need to match the subwoofer speakers, that is, commonly known as the "subwoofer".