Setup detail- Best to arrange things so as to place the center coincident-pair or single microphone forward of the baseline between wide-spaced omnis by about 20cm (8") if possible. Should form a shallow triangle arrangement as viewed from above or below.
If using angled directional L/R mics in place of the omnis, spaced less widely than the omnis would be, but at least twice as wide as a typical 2-channel near-spaced configuration, it helps to push the center position a good bit farther forward still. The forward spacing in the arrangement shown in the photos above, posted back in Jan 2020, works out well.