I have both, and agree with Ronnielee54. Also, IMO, the Sennheisers (being more laid back) work even better than the Koss for night-time listening (although both have taken me pleasantly to the edge of sleep many times).
It's typical of my non-systematic, side-grading audio experience that I have both ...
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Phones (in order of purchase):
Sony MDR71, Sony MDR51, Etymotic ER6, Panasonic RP-HJ50, Shure E3C, Koss PortaPro 2, Creative EP630, Etymotic ER6i, Sennheiser PX-100, Sennheiser HD555, Future Sonics Atrio M5