They could be buffered tape output jacks?
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Many pre amps/line stages include a tape loop. They will usually have jacks for Tape In and Tape Out on the rear. The Joule uses an isolated switching design for a tape loop and your second pair of jacks may be considered the same as Tape Out.
So long as there is not an impedance matching issue, they could also be utilized for a powered subwoofer or bi-amplification.
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