Update: So I went to remove the intake an noticed that the seal connecting the filter to the intake tube had not been secured flush.
Once re-secured, I took the car for a few runs and the sound was noticeably better.
I think there was a slight leak in the connection which was causing an abnormally loud intake "whistle", which was the sound that was so outputting.
The intake is now behaving correctly, and the sound is outstanding, understated yet still aggressive. So, unfortunately I am not going to be selling this intake given its sound is much more enjoyable when functioning correctly. Sorry to waste anyone's time.