It may seem like Matt Newport's talking up his audio referencing skillz, but he's just trying to share some advice culled from years of working with every kind of audio gear imaginable. He bought this RE204 to help target and eliminate distortion and other problem "specs" from audio equipment, but...
The De-Compressor increases the gain above the threshold. It can restore lost transients on over compressed audio. The De-Expander increases the gain below the threshold. It lowers the dynamic range in a different manner than a compressor.