Density conjecture for the square-grid sandpile

Let $b6_c$ be the threshold density of the fixed-energy sandpile on the square grid, and let $b6_s$ be the stationary density of the classical abelian sandpile on $b6^2$. Density conjecture. On the square grid,

$$b6_c=17/8.$$

This prediction says that driving and dissipation in the classical model push it toward the critical density of the fixed-energy model. The value $17/8$ is the rigorously estimated stationary density, while the equality with the fixed-energy threshold remains open.