The sub cables are not marked for directionality, due to the fact that the change in direction doesn't very audibly affect LFE. It's mostly audible in upper midrange.Therefore, plug it in either way, as both ways are technically correct. Feel free to experiment and see if one side sounds better to you.
AudioQuest
The Quest Group
If the wire does not have direction arrows, then the way you connect from AV amplifier to Subwoofer is not crucial.
The Velodyne Audio Product Support Team
Velodyne Inc.