Apple Macintosh Performa 6400 Series - Manual (Page 49)

Connecting external stereo speakers
You can take advantage of your computer’s stereo sound output by attaching
externally powered (amplified) speakers. When you attach external speakers,
the speaker in your computer changes mode and functions as a subwoofer.
The subwoofer produces bass sound to complement the stereo sound coming
from the speakers.
Subwoofer (internal)
Externally powered speakers
49 Connecting Equipment and Expanding Your Computer