With regards to Frank, the difference was immediately noticable after seeing Sean Kinney play with Alice In Chains. You can see Sean putting in effort and intention, even on the slow songs. He's driving the band forward. Then Frank comes on and he's just on autopilot, just one song after another. It's subtle but as a drummer myself I picked up on it pretty quickly. It's more of a body language thing I guess but it affects how the song sounds and how the band plays together.