The mono compatibility test
Fold your mix to mono (Utility plugin in Ableton set to mono, or check on a real Bluetooth speaker). If it sounds significantly thinner, duller, or quieter — you have stereo information that's cancelling in mono.
Where to add width (safely)
Width works best in the mid-high range (above 200 Hz). Stereo chorus on pads, double-tracked synths hard-panned, wide reverb returns — all safe. The key: keep the low end (below 120 Hz) mono or near-mono. Low-frequency stereo content almost always causes phase issues.
The Haas effect trap
Adding a short delay (1–30ms) to one side of a mono signal creates artificial width. It sounds wide in stereo but collapses in mono (the delayed copy cancels the original). Avoid this trick unless you've verified mono compatibility.
To fix this stereo problem, here are the tools PRISM producers reach for most — each picked for this specific case, not a generic list.