Ok, good. Now you are finding out what your system is physically doing. I'm going to give it more of a think but I'm interested in your sub now (Put a pin in the out of phase / in phase thing for now). I will defer to someone here who is a pro installer but I have a guess as to the problem. If the sub gets sound when both channels are in action but not when one is off that COULD, and I say could, mean it is receiving a signal from only the positive or negative wires of your amp/headunit and not a positive AND negative wire. You don't indicate if you have a centre channel or if your amp provides for one adn I'm not going to go search for your amp manual. The centre channel needs information from the left and right input to operate. Take one side away and it has no reason to give centre information. What I need to understand is why the sub would go out when the signal is out of phase. The next part is a huge guess - I am only skilled at installing the stereos I have done over the last 30 some years and I can't play with yours to find out where wiring is going. Ideally a sub uses summed mono information from the negative and positive signal coming from either the head unit or amp. It will use either left OR right or both channels to push signal the sub. That way you could have a bass playing in the left channel and still hear it from the sub. The amp should have a dedicated set of connections labelled SUB but it doesn't have to. The installer could have wired the sub in as part of the front speaker set. The guess is that it is possible you have the wires coming from a CENTER connection to your sub. The other guess is that the sub is miswired with signals from only a negative or positive set of connections. Both of those may cause the sub to go out when one side is playing. You don't mention if this is happening only with the fronts. I will presume it is. If you haven't done yet can you check to see if the same thing happens with just the rears? So here's the next step from my humble position. If you don't want to take it to the same or a different installer (I would imagine and hope the same installer would recheck their work no charge) or have a friend who is good at this stuff (or fly me down ) I would suggest taking your amp manual and walk through the wiring positions on the actual amp. You need to know your sub is being powered by the right connections. And this assumes the amp is operating properly. You have 3 weeks into tuning this beast and you seem dedicated to getting a good sound. Take your time and go through this step and you will at least know more. The good part is that your initial guess is right - there is something going on with the wiring that causes the audio problem you have. Does anyone else have thoughts? Am I completely off base with this path? As I said I am not a pro so I may be missing something about car amps.