JBL GX-A604 Owner's Manual page 21

Rear Crossover control: The Rear Crossover control setting you use for subwoofers will depend on the type and location of your system's
subwoofer. Start by setting the Rear Crossover control to a frequency somewhat lower than the 80Hz setting on the front channels. After listening
to music on the system for a time, fine-tune the Rear Crossover control setting to achieve a smooth transition from the subwoofer to the rest of
the system's speakers while avoiding a "hole," where the sounds that occur between the subwoofer and other speakers seem to drop out. The
illustration to the right shows the recommended Rear Crossover control frequency range.
GX-A604: 2-CHANNEL OPERATION
Input Mode switch: Set the Input Mode switch in the "2CH" position.
Front Crossover Filter switch: Set the Front Crossover Filter switch in the "ON" (high-pass) position. This will limit the amount of low-frequency
energy sent to the speakers, significantly reducing distortion and preventing the speakers from damage. Exception: If you have connected large full-
range speakers (6" x 9" or larger) that can handle significant bass to the amplifier's front channels and you are not using a subwoofer in the system,
set the Front Crossover Filter switch to the "OFF' (full-range) position.
Rear Crossover Filter switch: Set the Rear Crossover Filter switch in the "HP" (high-pass) position. This will limit the amount of low-frequency
energy sent to the speakers, significantly reducing distortion and preventing the speakers from damage. Exception: If you have connected large full-
range speakers (6" x 9" or larger) that can handle significant bass to the amplifier's front channels and you are not using a subwoofer in the system,
set the Rear Crossover Filter switch to the "FLAT' (full-range) position.
Rear Crossover control: When the Rear Crossover switch is set in
the "HP" position, the Rear Crossover control determines the frequency
where the low frequencies sent to the speakers begin to be reduced in
volume.
When running the amplifier in the 4-channel mode you should set the
Rear Crossover control according to the size of the speakers connected
to the amplifier channels – the smaller the speakers the higher you
need to set the control to protect the speakers from damage. After
initially setting the crossover frequency, listen to music with strong bass
that you are familiar with. If the speakers sound distorted or distressed, move the crossover frequency control to a higher setting to eliminate the
distortion/ distress. The illustration to the right shows the recommended crossover filter frequency ranges for different speaker sizes.
6" and 5" Speakers 4" and Smaller Speakers
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