I have been in touch with Josh about this, and can confirm that he DID upgrade his Squier with a preamp some time ago. He provided me with the name of the company which produces it: audereaudio.com
I had considered doing this to my own Squier before I finally decided to just upgrade the entire bass: Squier V.M. '70s Jazz Bass vs American Ultra Jazz Bass. . . 
HTH and all best,
Joe