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The only relation to Mineral that Pop Unknown has is that Gabriel Wiley, Mineral's drummer, became the drummer of Pop Unknown post-Mineral. Pop Unknown's singer is/was a fellow named Tim Lasater, while Mineral's singer is/was Chris Simpson. Simpson went off to form The Gloria Record with Jeremy Gomez, Mineral's bassist/ex-bassist. Simpson was the singer of The Gloria Record, not Pop Unknown.
Therefore: the guy's voice didn't get too deep - it was an entirely different singer.