Mark_MI am not some intense audiophile, so my advice is definitely from a regular users perspective. I am using the Magni 3 for the amp paired with a focusrite interface I was for the DAC.
On to my thoughts, I found the HD 6XX to not be a major groundbreaking jump. At low volumes it sounds very similar, but you'll find the HD 6XX packs a punch with the upper and mid volumes. Sounds more full and close, while the bass has a notable improvement. Overall a good step upgrade, but not something night and day for me.