GMC Sierra Denali 2006 Owner's Manual page 182

Setting the Tone (Bass/Treble)
AUDIO: Push and release the AUDIO knob until BASS
or TREB appears on the display. Turn the knob to
increase or to decrease. The display will show the bass
or treble level. If a station is weak or noisy, decrease
the treble.
To adjust the bass or treble to the middle position
while it is displayed, push and hold the AUDIO knob.
The radio will produce one beep and adjust the display
level to the middle position.
To adjust all tone and speaker controls to the middle
position, push and hold the AUDIO knob when no tone
or speaker control is displayed. ALL CENTERED will
appear on the display, you will hear a beep, and the
display level will be adjusted to the middle position.
AUTO EQ (Automatic Equalization): Press this button
to enhance the audio performance for different
passengers in the vehicle.
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The setting last chosen will appear on the display when
you first press AUTO EQ. Each time you press this
button, another setting will appear on the display and
AUTO EQ will switch to one of the preset settings listed.
The audio system allows you to choose from
four different equalization settings: normal, driver,
rear and spacious. These settings can be used while
listening to the radio, cassette or the CD player.
NORMAL: This setting provides the best overall vehicle
sound quality for all seating locations.
DRIVER: This setting gives the driver the best sound
quality.
REAR: This setting gives the rear seat passengers the
best sound quality.
SPACIOUS: This setting makes the listening space
seem larger.
The radio can save separate AUTO EQ settings for
each preset and source.