His room is half the width and 3/4 the length. He's got them smooshed against the side walls and back wall. I've tried placing them in various, impractical positions and have managed to separate instruments along the width somewhat but nowhere the degree that I can with M5s and M100s. I have never been able to get any depth.

Almost every recording sounds good with the M5/Pioneer combo. I can't say that with the M100/Onk combo. The culprit is the Onk because swapping to the Pioneer, I get similar performance as the M5s.