probably either grind / dose / tamp (does that narrow it down enough?!)
Do you adjust the grind for doubles vs singles? What is the shape of the bottom of your double vs single basket? Singles often have tapered bottoms, whereas doubles are often right angles.
Think of a flat roof with no supporting beams - as you pile weight on the top, it will bow toward the middle. A similar thing happens with pressing on the grinds when the bottom is flat and the top is level, which you may not see with a single because of basket shape / dose.
Try scraping the top with a circular object like your doser / hopper lid, to give a concave surface to the grinds (higher at the edges), and then tamp that.
Id be interested to see if it helps, since thats what I do with my VST, but I dont have a naked basket to see the effects :)