Dynamic range enhancer, can handle pro +14 dBm level, powerful linear compressor/expander, useful to compress listener program music to nice even level background, excellent audio quality circuitry similar to most expensive DBX equip., power supply cap upgrade, audio capacitor upgrade, calibrated, etc.
These machines are rare and special because they are true gain ranging compressors for engineers who want to squash a high dynamic range without much pumping. It is also able to expand dynamics, adding energy and depth to dead recordings.