As Narrod said, if your subwoofer has 'high-level' inputs in addition to the 'line-level' inputs you can run a second pair of speaker cables from the speaker terminals on your amp to the high-level inputs on your subwoofer.
That can be done either by running cables from the 'B' speaker outputs on the amp if you have them, or by running two sets of speaker cables from a single set of speaker terminals, one set to your main speakers and one to the subwoofer. If the second option is all that's available to you, attach the second set of speaker cables either by using banana connectors into the ends of five-way terminals or by doubling up the spade or bare wire connections, or a combination of any of those.
The second set of speaker cables can be a smaller gauge than your main cables, it's just carrying frequency and amplitude information that the subwoofer amp will work with.
That can be done either by running cables from the 'B' speaker outputs on the amp if you have them, or by running two sets of speaker cables from a single set of speaker terminals, one set to your main speakers and one to the subwoofer. If the second option is all that's available to you, attach the second set of speaker cables either by using banana connectors into the ends of five-way terminals or by doubling up the spade or bare wire connections, or a combination of any of those.
The second set of speaker cables can be a smaller gauge than your main cables, it's just carrying frequency and amplitude information that the subwoofer amp will work with.