using the auto on. Right now I have it turned to auto-on on the back. It is hooked up via a single RCA plug from the back of the A/V. I have it plugged in correctly and I double checked everything.

The sub is plugged into a separate outlet separate of the power conditioner.

Any ideas on what to do next?

Check power cord for poor connection/dog chew marks.
Check outlet for power with another product.
Troubleshooting guide in manual.
Call Velodyne.
Take/ship back to place of purchase.

the sub separate of everything else.

I would like it to come on when the A/V comes on.

My DD-18's auto-on often stayed off when I thought it should have enough bass content in the signal to power up. Sure enough, when I turned the sub on manually, no additional bass.

Still, my Velodyne DD-18 is a piece of junk. With the auto-on engaged, the woofer makes a slight but annoying pop with auto on. I now run with auto-on defeated. The amp also sometimes developed a loud AC hum that would go away (for a while) if I turned power off and back on. Finally after three months, the DD-18's amp just died altogether. I sent the 1250 watt amp and electronics (which weighed about a whopping 1.5 lb) back to Velodyne for replacement. The new amp has been working for about a month now, but I won't be surprised if it dies again.

Are you sure you dont have the sub connected to EXT in sub? and not sub OUT

I checked the A/V manual and it was set up correctly for auto-on....it is annoying have to turn on the sub each time even with a remote. I got everything down to one remote and the sub remote......