Hafler IRIS - Manual (Page 10)

Tape The tape MONITOR switches at first appear to allow only a
Monitoring redundant selection of what you hear from your deck(s) when
Switches you use the TAPE KEY(s). The important difference is that
MONITOR switches actually BREAK the audio path inside the
preamplifier, allowing you to listen to the deck’s OUTPUT while
its INPUT is being fed from any source selectable by the KEYS.
This feature is necessary for selectable by the KEYS. This
feature is necessary for real-time monitoring in three-head
decks, but can be of some value for signal-path verification in
two- head decks.
Engaging both tape MONITOR 1 and tape MONITOR 2 switches
results in the selection of tape MONITOR 2.
Make it a habit after recording to check that all six of the round
buttons are back OUT-unless you are using EPL or MONO.