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While it has that orange adapter on the end of the cable, the LG subwoofer is being powered by the LG receiver just like the other speakers. The speakers attach with two wires (positive and negative), and that's also true for the sub - it just happens to have a connector on one end instead of bare wires.
The good news is that your Onkyo sub can accept speaker-level inputs for the signal. I believe that you'll want to cut the wire running to the LG sub and use it to connect your Onkyo sub (keep in mind that splicing cables together is easy, so you won't be permanently damaging anything). The connector end will still plug into the LG receiver, and the now-exposed two bare wire end (from the end that you cut, and you'll strip the two strands to expose the wire) will connect to the Onkyo. You would set the "Frequency" dial on the Onkyo to the highest level (200 Hz) so that it won't filter the frequencies because the LG receiver is already doing that. The piece that I'm not sure of is how to connect that single speaker cable to the Onkyo, which uses two (left and right) speaker level inputs. I believe that you'd only need to connect to either the left or the right because all of the bass management is being done by the LG receiver, but hopefully someone else here can chime in to verify.
A
possible alternate approach is to not use that subwoofer output jack at all, but rather run the front left/right speaker cables from the LG receiver to the Onkyo subs speaker-level inputs, and then run speaker wires from the Onkyo speaker-level outputs to the front left/right speakers. I say "possible" because I don't think that you can change the crossover settings for the speakers in the LG system, which means that approach wouldn't work - but I put it here in case I missed that setting in the manual.
For other members here, this is an image of the LG sub with attached cable.
