I also tried some dedicated cables with 1/4 inch at one end and RCA at the other. Same result.

And into the line input, I tried an insert cable, being two cables joined into one stereo plug at one end. Same result.

Pa900 and Bose both plugged into same surge protector.
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Graham, Korg Pa1000, Korg G1 Air, Countryman E6, Roland BA330, 2 x Roland CM-30, , Mackie SRM150