Precision Acoustics HDS10 staying on all the time and making a humming noise

Hi,

I purchased a set of Precision Acoustics 5.1 speakers which included an HDS10 subwoofer. I also purchased a Pioneer VSX-820-K receiver to go with them. I have the sub connected with a subwoofer cable only. The crossover is set to 150 Hz (the receiver's crossover is set to 80 Hz) and the volume knob is set in the middle of it's range.

In the subwoofer manual it says that when the power is set to Auto "the amplifier will turn itself off to stand-by mode if it does not sense a signal after approximately 10 seconds." So I put it on the Auto setting.

The light on the front was red when I first plugged it in. When I turned on the receiver the light turned blue, I assume to indicate that it's getting a signal. However, when I turn the receiver off the light stays blue and after about 10 seconds I can hear this constant hum from the sub. If I switch the power switch to "Off" then the hum goes away and the light turns red again. After that if I put it back to Auto the light stays red and there is no hum. Sometimes I can even hear the hum when watching a TV program that has no LFE channel too.

So it seems that it's not going in to standby? Or, since the hum doesn't seem to start until after about 10 seconds maybe it's sensing there's no signal but not going into standby properly?

Has anyone had this issue before? Any idea how to fix it? Is the sub defective?

Thanks!

In the manual it says that for some content the signal might be too low for the sub to detect and turn on automatically. If that's the case it suggests leaving the sub turned on all the time. But if it's making that humming noise constantly then how could I do that? It seems to imply that the humming when not receiving a signal is abnormal and maybe not related too the auto-on... I used to leave my Yamaha sub on all the time (it didn't have auto on or off) and it never hummed when the receiver was off...

So my last post got me thinking. When I got home I tried a few things. First, I turned the subwoofer on with the receiver off and sure enough it made the humming noise. Then I turned the subwoofer off, unplugged the subwoofer cable, then turned it on again and there was no humming. Then I switched the subwoofer cable that I bought with the one from my old Yamaha system. I turned on the subwoofer with the receiver off and there was no hum! So, it appears it has something to do with the cable I was using. The one that hums is longer (15 ft) and thicker, could that have something to do with it? Or could it just be faulty? It's also made by Dynex, but I didn't think I'd need a top of the line subwoofer cable...