Using stereo wideners to increase loudness

Sounds that are closer to the listener have the effect of “wrapping around” the listener’s ears.

Mike Senior says mono recordings can sound narrower than the instrument itself, if you’re seated in front of it.^[ Mixing Secrets for the Small Studio pg. 153]

Conversely, sometimes an already stereo signal may cause Phase cancellation, in which in case, you may want to narrow them so they’re more coherent, as explored in Treating problematic stereo sounds in a mix.

In a similar vein, EQ boosts on the sides channel can increase the width in a frequency-specific way.

I use MStereo and Pro-Q 3 for this purpose.