Velodyne Sub Trouble

I just received a lightly used Velodyne HGS-18. It's less than one year old and apparently worked great for the seller. It arrived in flawless condition, but doesn't play. I initially wired it using the speaker level ins/outs. I hooked the speaker outs from the receiver to the speaker level inputs on the sub. I ran the speaker outs on the sub to my main speakers. All other settings on the sub were as specified in the manual. I turn it all on and get sound from my main speakers but nothing from the sub. I check the troubleshooting in the manual and do it all (power is on to the sub, mute is off, volume correct etc.) - still nothing. I've unhooked all the speaker inputs/outputs and taken a CD player and plugged it directly with the RCA cable to the line level input on the sub (rec. by velodyne tech support) and still nothing. What could be going wrong? Please help.