Deep Cut Friday The longest track on 1994’s ‘The Downward Spiral’ was a staple of the recent Peel It Back tour Nine Inch Nails singer Trent Reznor performs on stage, United Kingdom, 1992. (Credit: Martyn Goodacre/Getty Images) Each week, SPIN digs into the catalogs of great artists and highlights songs you might not know for our Deep Cut Friday series.  Nine Inch Nails’ The Downward Spiral and Soundgarden’s Superunknown were released on the same day in March 1994, and both blew my underdeveloped 12-year-old mind as two of the most dense, ambitious, and rewarding alternative rock albums of the era. The second full-length Nine Inch Nails album in particular felt like my introduction to music that sounded better on headphones, where I could better hear all the minute details of Trent Reznor’s harsh yet beautifully detailed digital soundscapes.…