One step closer. First, I started by adding a pull-down resistor between IN and G—no difference. Power still comes out of the LPS.
I then switched to an 80W LPS I had lying around, and everything works.
However, the next problem arises: I’m not getting any power from the laser tube. At 25 mA, it produces less than my 40W laser. I suspect the tube is defective, but it’s relatively new. My knowledge about this is too limited, though. Sigh.