I never thought about the Beyer DT 880 600 Ohms, would be quite a difference as the 424’s are 2000 Ohms and the A20 drive’s them with ease. I’ll have a think about them.I haven't used the Beyer amp but I've got 2 pairs of Beyer DT880 Edition 600 Ohms which should pair nicely with them I'm sure I could let one go, barely used in the box with the carry case say £80 plus postage.
I found them to have a very balanced sound compared to others (eg more natural than the V shaped curve on the DT990s) and they scale up really well.
I had the 424s many years ago and liked them but realised they were quite bright and lacking in bass when I tried some others.