I think you are too young to know about Koss and the history.
Koss made back in time classical friendly headphones, as well the Koss signature is still technical and highly PRAT oriented.
Koss HV and the Auditor Series.
They are one of the latest musical and soundstage oriented Koss headphones.
One of the fastest dynamics, precise with good attack and decay. The Koss A/250 make in my eyes rounds on the HD600. Better transient and texture response. Far more airy, lively, spacius with better soundstage and open seperation. Very fluid Mids but a bit too treble agressive. In comparsion the HD600 has a lot of muddy mid bass, don't extends very deep, is slow, but with smooth and more natural mids and silk highs. Koss shines here trough the bandwidth with transient energy, is more warmth, but also put a bit mory weight on the body.
Sennheiser are relaxed sounding headphones, but the KOSS takes you on the breathtaking waves of sound with a smile.
The koss Auditor sounds best with electronic, Chamber music, Jazz, Rock/Pop, movies, woodwind instruments, trumpets, horns and fast Mahler oriented music. The sound signature of the Auditor is fully on the classical HV/1a (the Rock headphone) oriented, but also a mix of the HD6x0 and AKG 601.Some head-fier said, the Auditor sounds like the ESP/950, but the roots are still in the balanced HV Series. Maybe the HV Series is based on the first Koss ESP. If that is the truth, Koss made a good job.
The closed version of the Auditor line is the A/130. Up to date the best one closed Koss headphone for music. But needs a bit of modding.
The Reference series still sounds good, but is to harsh and to treble oriented. For music still better, than the modern pro4, but i can't really recommend these headphones. The build quality is also flimsy.
If you only heard Koss up to pro4 series, you missed really Koss. Only few pro4 revisions sounds really good, but these headphones are studio oriented.
If you want really go into the sound of Koss, start with the HV/1a.Thät's open your mind.about hearing is believing.