It is diffucult
You can follow my progress on this graphs. I did the calibration with check levels and the spl reader measures 75dB in my livingroom. First graph is how it turned out. Then i adjusted the filters and made a new measurement. To get the targetline back to 75dB again I did the checklevels routine with adjusting the receivers volume so the spl reader measures 75dB and rews input volume -18. When adjusting input volume withs rews input volume ctrl nothing happends..so I raise Line in in recording...Then I measure again and thats the graph number 3. I´m getting the same results as the way I did ´before..Resulting in the graph where you thought my mains where connected...