Lets say I have a headunit with 3 preouts, F/R/Sub. And I have a single 4 ch amp. But I want to power my front stage, rear stage, and my sub. Would it be possible to power the front and sub using the 4 ch amp. Then use the internal amp in the headunit to power the rear?

I tried and wired everything up. It doesnt seem to work. I just want to know if this is possible.