The main thing was using a reverb which mixes a little of left with right and vice versa, which gives a wide (3D) stereo effect.
ANWIDA reverb and Freeverb* (both free) do that L-R crossover thing, (whether you like it or not), Gverb doesn’t crossover, (which isn’t necessarily a bad thing).
As I mentioned I also boosted the higher frequencies which improves intelligibility, (but also makes any hiss noise more obvious).
[* apparently there are Freeverb2 and Freeverb3, the one I’ve got is “Dreampoint Freeverb”, no number.
Be careful where you download from: preferably only download from the manufacturer/creator].