I don't know about any problem with an output??
I have the OEM fader control set to the middle. Then I adjusted the new amp's volume to match the front speaker's volume. So with the fader control in the center, the front and rear volumes are equal.
It's a bit funny. If I move my head back just a bit, the rears are louder, if I move forward, the fronts are louder. So, they are pretty equally adjusted.
If there is more bass in the music, it comes mostly from the rear. Kind of fun!