The modifier stack in 3ds Max doesn’t work exactly like a layering system, you don’t have an overall contribution amount control for each modifier. Some modifiers are highly sensitive to topology changes below them, and Edit Poly is one of those. It you change the vertex count below , an Edit Poly is going to freak out.…
You could use Morpher to reduce contribution like Fabi_G suggested… Snapshot the mesh with the Edit Poly, delete the Edit Poly, add a Morpher, load the snapshot as a target, and adjust the morph target strength.