strange issue with my subwoofer

When I turn down the headunit volume to almost nothing at "1" my subwoofer is still pounding at whatever level I have the gain set at. I have to turn the headunit vol to "0" in order for the subwoofer to go out or I have to set the gain all the way to "0" on my amp but if I do that than I guess there's really no point of even having subwoofers in my trunk. I thought I was having a grounding issue so I changed the grounds to couple different spot but its doing the same exact thing. I just have a basic setup.. running rca sub out from my headunit to my amp. I also want to point out that my headunit does not have a subwoofer control. Can anyone please tell me what might be causing this?

it sounds like it may be a problem with your head unit. I would suggest returning the one you have for a new one (if possible) or maybe spending some money to buy a little more expensive headunit with a sub-output control.