Senn(heiser) HD 600, named the Huh Duh Six Hungeos by Wade, are a pair of open-back headphones, dating back to the 90s that he uses as a reference for neutral sound in headphone comparisons. He uses them because they have a very neutral tonality and a lot of audiophiles already own a pair due to their huge popularity, so they make for a good point of reference. They are used in pretty much every video he makes reviewing headphones, and a recording of them playing Walk Through the Park by TrackTribe through the miniDSP HEARS is used to describe what music should sound like before switching to a recording of the headphones being reviewed.
The full title of the headphones (as revealed in Supermarket Headphone Special) are "The Huh Duh Six Hungeos from Old Mate Senny Senn" (Senny Senn referring to Sennheiser)