Have anyone tried that?
select Pioneer models DEH-P3300 and up
have this feature.
All these HU are rated at a maximum of 50 W X 4
But you can use the rear channels to power a small sub or at least that's what they say. So the Unit Will become a 3 channel HU ( 70 w x 1 + 50 w x2).
But you have to connect a 2 ohm sub to the rear channels.