It would sound like shit. Driver and baffling designs for open- and closed-back headphones are different. So is the tuning.

They're not interchangeable.

As one wise poster in here said, put your hands over the cups of your open-back headphones. Sounds like shit, right? That's what it would sound like if you covered the grills.

P.S.: Your calculus of impedance is way off. High impedance makes a can tougher to drive, not easier. But sensitivity also plays just as important a role. The lower the impedance, the easier to drive. The higher the sensitivity, the easier to drive. Check both stats before determining if your source can drive a headphone without an amp.

I owned the 560s. They sounded like shit and lacked volume from a phone. They really need an amp -- even a portable one -- to sing.