So I had a bike fit (on my single bike). My intent was to try to migrate the saddle position & bar position to our tandem.

The fitter changed my single pretty radically. Cleats on shoes moved from far forward to far back. Saddle moved down & back. Bars moved up & back.

I've ridden the single on several short rides. It seems to be more comfortable initially but I find that the new position takes weight off my shoulders/arms/hands and transfers the strain to my lower back. My "core" has always been weak and this new position makes that more obvious. At the end of a short ride, my hands are no longer numb but my lower back is really sore. Aside from doing exercises to strengthen my lower back & abs, I'm just trading one complaint for another.