How to explicitly assign the position in Labeled[]?

You can have finer control on positioning if you use Legended in stead of Labeled:

Legended[DensityPlot[x^2 + y^2, {x, -1, 1}, {y, -1, 1}, 
   PlotLegends -> BarLegend[{Automatic, {0, 2}}]], 
 {Placed["a", Bottom], Placed["b", {{.85, 1.02}}]}]