speaker cable for bass

Copper is generally very good especially 14 gauge or lower. You definitely need a wire from conductive metal. Stranded is good because it is more flexible and less chance of breaking. Apart from that don't sweat about this issue too much. A subwoofer does three octaves at the bottom of the range where your hearing is least accurate.

Whatever you choose, I would avoid Nordost wires if bass performance is top priority. I would suggest XLO cables as an alternative to the above.

I agree with Ryder - XLO cables, even their cheaper speaker cables such as the Ultra 6, have tremendous bass performance.