defroster interference, remote bass knob

All of theses features you are talking about are mostly just filters.  Dont use your bass knobs, they just replace bass bost on your amps, and usually when used produce distortion.  Use only one of the filters.  For example,  adjust the subwoofer filter on the headunot to the highest setting probably something around 400hz.  ditch the bass knobs complety and then use the filter on the mono amp, set this at 90hz and adjust your gains.  This should give you much better bass. 

I would say it is more like a Eq that boosts the low frequency to the mono amp when hooked up..  Your gain should not be turned all the way up unless thats what is matched to the voltage output of your headunits preouts.  The gain is not a volume control, it is used for matching the voltage of your preouts.  With all the components set to 120 hz you will have some overlap, more or less depending what decibel slop the crossovers are using.  If the rears are distorting more try making their crossover point a little higher.  You are right as they arent really need that much.  Many people dont use rear speakers at all, they think is messes with their staging and they dont like how it sounds.  Components should help with your front staging and will generally play down to a lower frequency. You will also get more volume out of the components assuming they are set up right.  As for the bass knob once a system is set up right there is no need for this.  Leave it out at all costs it will just create more problems in the future.