Knuckles made numerous popular Def Classic Mixes with John Poppo as sound engineer, and Knuckles partnered with David Morales on Def Mix Productions. His debut album "Beyond the Mix" (1991), released on Virgin Records, contained what would be considered his seminal work, "The Whistle Song", which was the first of four number ones on the US dance chart. The Def Classic mix of Lisa Stansfield's "Change", released in the same year, also featured the whistle-like motif. Another track from the album, "Rain Falls", featured vocals from Lisa Michaelis. Eight thousand copies of the album had sold by 2004. Other key remixes from this time include his rework of the Electribe 101 anthem "Talking with Myself" and Alison Limerick's "Where Love Lives".