The HD 600 has been the headphone other headphones get measured against since 1997, and the reason is narrow and real: its midrange is close to neutral across the range where voices and acoustic instruments live, with none of the scooping or presence spikes that flatter a demo and annoy you later.
This is the one pick that will not work without an amp. At 300 ohms and 97 dB, a phone runs out of voltage well before the headphone runs out of dynamics, so it sounds quiet, soft and flat straight from a laptop. Fed properly it snaps into focus. Plan for a desktop amp or a powerful dongle at the same time as the headphone.
The compromises are the other side of the same tuning. Sub-bass rolls off, so this is not the headphone for bass-led electronic music. The stage is intimate rather than cavernous, which some listeners prefer and others find closed-in. Clamp force out of the box is strong enough that the first fortnight is uncomfortable on a wide head. Against that, Sennheiser still sells pads, headband cushions, grilles, yokes and cables individually, which is why decade-old pairs stay in service instead of being replaced.