wiring problem or bad head unit?

I got a VR3 head unit for rmy 4 channel amp.  Wired it with RCA cable, 2 outputs to the front, 2 outputs to the rear (subwoofer bridged).  When I played it, fronts played fine, sub played nearly nothing.  So to track down the problem, I switched cables at the HU so that the subs are playing out of what used to be the fronts.  Sub sounds great!  Now fronts are quiet.

Ok I figure  it is the HU, bad rear amp..  Bring it back and get a new one.  Same thing, exactly! 

Now, maybe both of the head units have blown the rear anps but I figure probably not,  Probably I have done something wrong.  But what could it be?

One other thing you may want to check is the actual HU settings on some headunits the rear RCAs are actually selectable between operating rear speakers or subwoofers if this is set wrong it could be causing your problem.

--->Richard

Thank you Richard! 

After reading your response I cracked open the manual and there, step 12 of the operation section. was:  How to access the "Subwoofer on-off control."  You have saved me a lot of surffering....  except of course for the now inevitable self-incriminations...

You are a true reason for the internet to exist.

Thanks,

Putts