This may be totally unrelated but it happens with me and my HD650 and DT990 600 ohms. Because my hearing isn't the same for both ears (unfortunately). I mean it's very close but my right ear picks up frequencies slightly higher than my left. I can tell this with symphony triangles and tambourines off in the distance--one ear picks up the nuances and shimmering sound of those instruments and the other (left ear) picks up just the striking of the instrument making the shimmer of the tambourine sound more like the closing of a high-hat from a drum kit.
So the conclusion is (for me at least), since the HD650 aren't as detailed as the DT990, the HD650 sound a bit off balance compared to the 990. I think the 990 frequency extension compensates for my slightly dulled hearing in my left ear. The same thing happens to the same ears when I switch the L/R cups on my ears.
So it's all in my head.