This is all good and fine for someone who has the budget and experience, but keep in mind that a decent amount of people joining this forum are budget oriented and really arenāt looking to spend around 500 or less on a headphone setup just yet.
I know some people who definitely prefer the 600 to the 4xx, and vice versa, its largely preference and use case. Comparing to the 4xx, the 600 is an upgrade in mid-range timbre, detail, and refinement/coherency, but a downgrade in soundstage, bass and treble extension, and speed. You have to figure out your preference before you choose. The 500 is a more direct upgrade to the 4xx
The 4xx is a great entry point and I think itās pretty worthwhile buying to try things out and go from there as you say. I just really canāt nitpick at the 130 new or even lower you can find used tbh