LP filter for audio measurement on Class D amp output

This is a LP filter with extremely sharp cutoff above 20 kHz to enable accurate measurements of Class D BTL amp output to speakers. Without such a filter, there is plenty of switching waveform left at speaker terminals despite internal filters . Rejection at 300 kHz switching freq is 80 dB, and at 1 MHz 120 dB. It has passive filter components + one instrument amp like AD 620, AD 8221, etc.