Well, thoughts are, can this amp be a unit that disables the pre-outs when the speaker outputs are in use, or does it have a setting that's required to be turned on to activate the pre-outs? Besides checking the manual for some obscure setting requirement, try things one at a time. Run the sub from the pre-outs without the speakers connected; if no result, connect it with the speaker wires to the speaker level inputs to see if it's even working correctly.

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Enjoy the music, not the equipment.