I found the solution for my problem with the low hum noise. After literally years of searching for a solution, opening my headset, isolating wires, testing different soundcards... I got the idea from a reddit topic...

I am in the same position. Especially because the LMH6643 is a low power op amp there might be compatibility issues with replacements. I will report if I find anything.

I am also thinking about the op amp modification. Did you try the upgrade or did you find any other reports about this upgrade?