Apparently 800 mA (the default setting in Marlin) was too high a drive current setting for the Y-axis motor in my AM8 (and the extruder motor, too, which was getting hot to the touch). Subsequent experimentation on the X and Z axes suggests that 200 mA should be sufficient for reliable operation without burning things up. I’ve also read that the extra-quiet mode (StealthChop) on the stepper drivers I’m using uses more power than the normal, not-as-quiet (but still not too bad) mode.
Fortunately, I have another printer at the ready to produce a replacement motor mount. It’s already sliced…just need to go home, load purple PETG into the Hypercube, and hit “start.”