sub pre amp output

You are right, the OEM amplifiers are generally not designed to be used with after-market head units.  You cannot just use "RCA plugs for connection ease."  The amplifier requires a specific level signal that is provided by the OEM head unit.  This is the case for most OEM equipment.  Sorry.Support the12volt.com

Cool, this must the case with the other amp'd subs I have then, the store bought units.  They also have zero gain.  It is almost like there needs to be an amp installed between the hu and the subwoofer to boost the signal into the sub, in addition to the amp on the sub.  I'm baffled.....this seems to me it should be plug and play, but it isn't.  I am talking about the JVC to the non oem amp'd subs, even the Sony hu before that did this. 

Thank you, I'll have to get the list of components I am trying to use.  And I agree, they should be working, but they are not.  This is why I am baffled, I plug the sub preamp out to the amp'd subwoofer and really, nothing happens, adjusting the levels on the amp have an affect, but it is minimal, the rest of the stereo drowns out the sub bass.