i did the same and now I have hd600 to compare to the 400i and the choice is a very difficult one. 650s were muffled as hell for no reason, the 600s have a clear sound but are still smooth as butter like 650s... this is a very special sound for me.

they have a compact image with layers detail while the 400i is like a very wide, flat canvas for detail. 600 sounds magical while the 400i sounds surgical, both impressive but 600 is pure enjoyment.

bass is second biggest difference. its smooth, punchy and natural... nothing like the clean, fast, detailed 400 bass. this is the hardest choice... rolls royce luxury bass vs ferrari performance bass.