Help with signal path

[achn2b 0]

I'm running two Sundown amps, now discontinued, an SAX 50.4 for front coaxes and then lineout from that into an SAZ 1000D for the subwoofer. One of the front inputs on the 50.4 broke off, so for a temporary fix, I just ran the outputs from the stereo into the rear inputs. It was just the other day however, that I noticed I no longer had any bass coming from the sub. 

 

Now before I go tearing everything apart to see what's wrong, my first instinct was to assume that the line output of the 50.4 receives it's signal from the left and right front inputs. So if I'm using the rear inputs only, no signal to send out to the sub amp. Would this assumption be correct? 

 

I really need to send the SAX 50.4 out to be repaired, but I don't want to be without music in the car for that period of time.