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Finally achieved accurate results from the expander. I did some research on how expanders are implemented electronically, and the peak envelope detector is normally implemented as a diode and capacitor + load. The discharge rate is constant so I updated to the code to do the same, and this fixed the dynamics issues I was hearing.
I created a test tone sequence that matches the IEC 61054 spec (Annex A - Encoder transient measurement method) and recorded this to a tape. I compared the VCR playback output to hifi-decode's output and they match almost identically. I think the low frequency rippling was introduced at some point during the recording, since both the line out and hifi-decode have the same shape.
I also added a few optimizations to the post processing code.