Boss BR-1600CD Owner's Manual page 202

Using a metronome
4. Move the cursor to "Phones2" under "Metronome" and
use the TIME/VALUE to set the output to be routed to
PHONES 2.
Phones2: OFF, ON, METRONOME ONLY
OFF:
The metronome will not be output via PHONES 2.
The audio form input, playback sound on the Tracks,
drum/bass patterns and Loop Phrases will be output via
this output.
ON:
The metronome will be output via PHONES 2.
At the same time, the audio form input, playback sound
on the Tracks, drum/bass patterns and Loop Phrases
will be mixed output via this output.
METRONOME ONLY:
Only the metronome will be output via PHONES 2.
The audio form input, playback sound on the Tracks,
drum/bass patterns and Loop Phrases will not be output
via this output.
At the time of purchase, both LINE OUT/PHONES
1 and PHONES 2 will be set to "ON."
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