Tampers

Tamper size has to match your basket, not the other way around. Most Breville and Sage machines run a 54mm basket, De'Longhi sits at 51mm, and commercial-style or E61 setups run 58mm. Get the size wrong and the tamper either rocks in the basket or won't seat flat, and either way the puck tilts before you've started.

Both tampers here are spring-loaded, so the puck sees the same pressure shot to shot instead of whatever your arm delivers that morning. One has a flat base, one's rippled. The honest answer is neither fixes bad distribution on its own. Pair either with a tamping station so it's not sitting loose on the bench between shots.