sub pre amp output

OK, I must be totally out of this whole stereo thing from the way things use to be.  I have a JVC HU with a pre-amp sub output and built in crossover...hooking this to an amp'd subwoofer, I get very little in the way of sound.  It finally occured to me that the sub output of the stereo is line out, which to me means I need an amp with a line in and a line out to my amp'd subwoofer?   The amp on the subwoofer will not amp this line out signal since it has no line in.   Question, is this close to being on track or not?   In short, is what I need a stereo, sub amp, then the amp'd subwoofer?  Thank you! 

Let me try to explain this better...I am using a JVC HU and an oem amp'd subwoofer.  The HU has a sub preamp ouput, but there is little to no audio at the subwoofer, this was also true with another amp'd subwoofer  I had and a Sony HU.  Same problem, different setups.   Do I need to amp the output from the HU before going to the amp'd sub?   I would think the amp'd subwoofer should take care of this? 

How have you connected a signal to your OEM subwoofer?Support the12volt.com

This is actually an 06 Explorer and an oem Explorer amp'd subwoofer.  The HU is not the Ford unit as stated, it was a basic no frills unit.   I don't think I follow your suggestion, but are you saying to just wire the sub to a left or right rear channel and not use the stereos sub output? 

Yes, from the preamp sub output from the hu and there is audio, but very little, definitely not any amping going on.  This has been the case on 3 amp'd subs and two different hu's with preamp sub outputs.   I am missing a piece of this audio puzzle.   I think I am needing an amp'd input to the amp'd sub, kind of like a driver.  This is what I am not sure of.  Basically the sub output to an amp, then to the amp'd output to the amp'd subwoofer. 

What signal from the head unit are you connecting to the OEM subwoofer amplifier, and hoe are you connecting to the OEM subwoofer amplifier?Support the12volt.com

I am taking the HU sub preamp output to the oem subwoofer via RCA plug terminated audio cables.  

brockdog08 wrote: I am taking the HU sub preamp output to the oem subwoofer via RCA plug terminated audio cables.

So the OEM amplifier includes RCA inputs?  That is unusual.  Does it include a gain control?  If so it is not set to the proper level.  Have you provided power to the OEM amplifier?  Also, have you turned on the sub output from the HU and set the output level?

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I added the RCA plugs for connection ease.  I have the OEM amp powered and the sub level on the hu turned up to 6, max is 8.  There are no adjustments on the amp since it is a hidden behind a panl type amp.  Even on the other two amp'd subwoofers I have that have RCA plugs, this symptom is identical, very low audio from the sub.  Here is a silly question, are there amps made to specifically amplify sub preamp outputs such as these on headunits?  Maybe these sub amps are not designed to amplify the line out and amplify an already amplified signal???