"Either side of common" is common ground. Read on and reference the schematic as you read too. Look at the P4 connection on the schematic and observe what is on each side.
To find the GNDA reference it is found all over the place on the schematic. In this case look at the power supply part of the schematic where it shows TP3 TP4 etc... On the PCB board you will find a center tap part of the transformer that is connected by way of the P4 faston. That is essentially ground or GNDA, and also you can pull out the DMM to test continuity to ground. Do the test for continuity to ground with the amp off to become familiar with all these point that reference back to ground. The chassis is a reference to ground as you will soon discover. I would get some alligator clips to clip the black probe of the DVM to chassis ground. Look on the schematic and find common grounds to double check it is connected securely. Look for the AC safety ground wire that is bolted to the chassis... That is ground reference point. With amp off clip one probe on P4 and touch the other probe to safety ground clip on the chassis. Do this with the continuity setting of the DMM and you will see .002 or something very close to zero ohms. Most DMM's have a beeping noise that lets you know there is continuity too. So, what are you noticing?
You will need to have the amp on to test for voltage. So secure that black probe to chassis ground and then use red probe to touch the test points to read voltage. Read as much as you can about safety tips before you proceed testing a live circuit, especially if this is your first time.
To find the GNDA reference it is found all over the place on the schematic. In this case look at the power supply part of the schematic where it shows TP3 TP4 etc... On the PCB board you will find a center tap part of the transformer that is connected by way of the P4 faston. That is essentially ground or GNDA, and also you can pull out the DMM to test continuity to ground. Do the test for continuity to ground with the amp off to become familiar with all these point that reference back to ground. The chassis is a reference to ground as you will soon discover. I would get some alligator clips to clip the black probe of the DVM to chassis ground. Look on the schematic and find common grounds to double check it is connected securely. Look for the AC safety ground wire that is bolted to the chassis... That is ground reference point. With amp off clip one probe on P4 and touch the other probe to safety ground clip on the chassis. Do this with the continuity setting of the DMM and you will see .002 or something very close to zero ohms. Most DMM's have a beeping noise that lets you know there is continuity too. So, what are you noticing?
You will need to have the amp on to test for voltage. So secure that black probe to chassis ground and then use red probe to touch the test points to read voltage. Read as much as you can about safety tips before you proceed testing a live circuit, especially if this is your first time.