JBL GX-A604 Owner's Manual page 16

GX-A3001 SPEAKER CONNECTIONS
Minimum speaker impedance; 2 ohms (single subwoofer); 4 ohms (2 subwoofers)
The GX-A3001 has two parallel sets of speaker connectors, allowing you to connect two subwoofers.
4-Ohm Subwoofer
• If you are using a single subwoofer you can connect it to either set of GX-A3001 speaker connectors.
• If you are using two 2-ohm subwoofers you can connect them to one of the sets of connectors in series to be above the required 2-ohm minimum
impedance.
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Fuses 20A +12V
4-Ohm Subwoofer