If you went through all the trouble why did you not use a volt meter and a all bit (0dB) sine wave. Just run the the sine waave at max volume and read your voltage out with a meter.
Line level signals are like 2 mV to 8 v for a Car Audio Head unit. Normally anything over 4 volts is a modified HU.
8 V to 16 V out the HU is speaker level outputs. What you are reading are to outputs of a bipolar amp not a simple opamp device.
Just because it plays your speakers low does not mean it is a low level. It may just mean it is a WEAK amp in the HU and it can't push enough current for the volt it has given speaaker impedience and flux load.
That is, no power for the load, not a low level signal.
