I believe I see where you are getting hung up Amir. You are intent on getting 4Vrms into the analyzer. It appears that the designers did not account for your desire to test exactly at 4Vrms. The Amplifier Sensitivity setting defines the clipping point of the amplifier. There does appear to be a mistake here. The Amplifier sensitivity setting allows a range of 0.1V - 7.0V. However, anything over 4.0V does nothing.
I was able to get exactly 4.0Vrms by setting the HTP-1 volume to +1dB and the AP to -0.712dBFS.
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You are not seeing any sort of DSP compression, you are clipping by setting volume to +10dB. If you want to ensure you reach 4V simply set volume to +1dB and test. With a volume setting of 10dB you shouldn't exceed -10dBFS to avoid clipping.
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