Multiple distinct techniques for widening stereo use different mechanisms and have different mono-compatibility profiles

Five main approaches to stereo widening exist, each with different properties: (1) Fake double-tracking — duplicate the track, rearrange repeated sections, opposition-pan; very effective, fairly mono-compatible; (2) M and S (Mid-Side) processing — encode stereo to mid+sides, process independently; surgical control but can cause severe mono cancellation if pushed; (3) EQ-based widening — invert EQ settings between left/right copies, spreading spectrum across the panorama; moderately mono-compatible; (4) Pitch shift widening — slight pitch detuning between left/right; mono-compatibility depends on implementation; (5) Haas delay widening — pan dry and single echo to opposite sides within approximately 30ms; risk of mono comb filtering. All techniques must be checked in mono.

Examples

Fake double-tracking example: a chorus has a chord pattern that repeats every 4 bars. Duplicate the guitar track, swap the two 4-bar sections in the copy, and pan the original left and the copy right for instant width with minimal mono penalty.

Assessment

A mono bass guitar recording needs widening. Which of the five techniques listed would you NOT use, and why? Which technique provides the most mono-compatible widening?