Correct way to test preamp voltage with m

Set your DMM to AC volts.  Click on the http://www.jlaudio.com/tutorials/index.html# "Gain-Setting Tutorial".  Notice the speaker connection to the amp terminals;  then on into the tutorial, take notice of the multimeter reading when the probes are placed onto the speaker terminals of the amp.  With the gain all the way down, the meter is showing a reading in AC volts of 4.1.  This represents the preamp voltage that the amp is receiving.

In your case where there is multiple amps, the reading you get depends on how the signal is distributed to the amps.  Read AC volts at the first amp in the chain, with other amps disconnected.  If signal is split with Y-splitters, disconnect the other amps.  You should get a fairly accurate reading of AC volts from the head unit, provided you are using a good test tone CD as shown in the JL program.