Yamaha HS8S Features:
- 8" bass-reflex powered subwoofer delivers low frequencies down to 22Hz
- 22Hz - 150Hz frequency response
Yamaha HS8 Active Studio Monitor Features:
- High-performance drivers and mounting system
- 8" cone woofer, 1" dome tweeter
- Impressive 38Hz to 30kHz frequency response
The sub would fill in from 22Hz to 37Hz, where the HS8's don't reproduce that low of a signal.
A friend that plays Bass, has the HS8's and the HS8S sub. If you play bass or like low kick drums, you'd appreciate the sub. For just guitar and drum tracks, HS8's are plenty, depending on your room.