Sound loss

Hi, I have a Alpine CDA9856 hu rf cpapacitor feeding a Infinity 475a for four speakers. In addition a RF 550 punch driving two 10' dual coil speakers. The front doors are Inf. component rears are Inf 5x7's. power led at amps are showing power. capacitor shows 12 volts. The sub lost power to speakers and currently the driver side component spker works and the pass. tweeter (only) works. Can I test for voltage at speaker terminals at amps for output? Do I use pos. prong at terminal and ground other prong to Chassis or amp? what voltage readings should I get? Can I test head unit at rca out/in?