Yes I agree. In terms of comfort, the DT880s are *much* more comfortable - even luxurious - out-of-the-box than my HD6XXs (because of the clamp force), but my 8-year-old HD650s are more comfortable than both - there's no squishy clamp force on them after years of use. The main point of difference (when clamp force is no longer an issue) is the Sennheisers have the nice foam on the headband so there's very little pressure on the top of the head, which I get from the DT880s on occasion.
I've found that even my 250ohm DT880s need more volume than the HD650s, which of course are rated at 300ohms - I'm not sure what mechanics are involved there to cause that, but it's true. I can only imagine how much grunt it would take to get decent volume and fidelity through to 600ohm cans.
But either way, they are very different cans from each other and that's why I own both (as well as several other brands and models, like most of us here do haha) - I love mixing things up for my ears and hearing different things from the music I'm listening to.