Brand new. Opened the box to check the contents, plugged them in to check everything works. It does but they’re not punchy enough for me to use with my bass.
Here’s the blurb from Sennheiser :

Topping Sennheiser’s 500 series, the full sized premium headphone offers near-audiophile levels of sound quality and exceptional wearing comfort thanks to the large ear cups and soft replaceable ear pads
The open back HD 599 SE delivers impressively natural spatial performance.

Sennheiser’s proprietary transducer technology

Powered by Sennheiser’s proprietary transducer technology including the “Ergonomic acoustic refinement” (E.A.R.) design, which channels the audio signal directly into your ears.

Contemporary design

Featuring a contemporary design that is both attractive and ergonomic. A real treat for all, who listen to their music with a sense of discovery. Get the latest edition of the legendary Sennheiser sound with these brilliant headphones.

With the HD 599 SE you step into the world of audiophile sound.

Outstanding natural spatial performance

There you will experience outstanding natural spatial performance, brought to you by high-end proprietary Sennheiser transducers. You may have listened to headphones that are quite good. The HD 599 SE will show you what lies beyond that. Be warned: there is no turning back.

Also the comfort is superior

The HD 599 SE features an open-back over-ear design. 

Large ear cups with soft replaceable ear pads

Thanks to the large ear cups with soft replaceable ear pads there is no pressure on your ears, only sound. So nothing will stop you from listening to track after track, discovering what the HD 599 SE will show you that you haven’t even heard yet.

SpecificationsArticle number (SKU)508697Ear couplingOver-EarAcoustic principleopenAdapter1: 6.3mm 2: 3.5mm detachableCable length1: 3m 2: 1.2mWeight250 gFrequency response (speaker)12 Hz to 38,500 HzImpedance50 ΩSound pressure level (SPL)106dB SPL (1kHz/1Vrms)Total harmonic distortion (THD)< 0.1% (1kHz, 100dBSPL)Wearing styleHeadbandTransducer size38 mmActive Noise CancellationFalseShow lessWhat's included

• HD 599 SE headphones
• 3 m cable / 6.3 mm straight plug
• 1.2 m cable / 3.5 mm straight plug
• 6.3 mm to 3.5 mm adapter