The 'shutdown' voltage for the source unit and amplifiers will be below 12 volts but because voltage tends to sag when under load you need to measure at least 12 volts for testing purposes. A standard, fully charged, good/new condition, flooded battery will rest around 12.7 volts. You just want to maintain a voltage at your amplifier and source unit inputs of within a tenth or so of the battery terminals. If you a have a one volt or more drop for example, then you know you have a cable and/or connector issue do deal with. You should not be rewiring anything until you have taken measurements. A simple multimeter put to task will expose the problem and replace hours of trial and error.