I recently purchased the D600's and after a few of weeks of almost non-stop listening to them, can say they are hands down the best headphones I've ever owned.... or heard.
They have the tight, low end bass response coupled with good mids and highs I've been searching for in my ideal headphone. While my AKG 701's may have a wider soundstage, and better instrument separation, they lack the bass response the new Denons deliver.
Its a joy to re-discover the entire lossless music collection. Paired with a Cambridge Audio Dac Magic and Little Dot MK V headphone amplifier, they deliver stunning fidelity. Music you thought you knew, comes alive with unprecedented clarity.
I find they work equally well with all genres, comfortably and effortlessly. They are also the most comfortable pair of headphones I've ever worn. There's nothing bad I can say about them, only wish I had a pair of Denon's earlier as I realise now what I've been missing.
Since purchasing these, I've sold the Sennheiser HD650's which, inspite of silver cable upgrades and endless burn in time, retained the muddy "Sennheiser Veil" that sounds like you're listening to the music in the other room !.
To me these Denon AH D600's sound a bit like my old B&W Nautilus speakers rather than headphones. After a while, the phones "dissapear" and you are immersed in the music itself. I sometimes find myself taking them off to make sure the A/V Amp is actually switched off.