http://forums.sandisk.com/t5/Sansa-Fuze/Is-earphone-impedance-critical/td-p/120982
Sennheiser claims compatibility with mp3 players.
"Widely Compatible
The Sennheiser HD 280 PRO is compatible with MP3, iPod, DVD, and CD players as well as mobile phones supporting a 3.5mm jack plug. HD 280 PRO also supports (listen only) the latest generation of tablets, from the iPad to the most recent Android tablets and the Blackberry Playbook. .
But you might have to crank it up a little.
I use 32 ohm Grado headphones with no problems. Grados are open, not closed back, but they sound excellent.