I'd want to know (a) if it was an analog or digital filter, (b) if the >20kHz signal intensity was normal/plausible and (c) how they ensured that the playback s

I'd want to know (a) if it was an analog or digital filter, (b) if the >20kHz signal intensity was normal/plausible and (c) how they ensured that the playback system wasn't generating intermodulation distortion products.