Quote Originally Posted by Geoffcin
You have to be careful matching the application with Sennheiser, or any other audiophile grade headphone. Most of them are 300 ohm phones, and as such will do MUCH better with a headphone amp. The little op amps in MP3 players will either not have enough gain, or be pushed to stress trying to drive them. If your looking for a good can from Senn that will work with these players then you gotta go with the 595/555 series. Or even lower down the chain a Senn HD-280 pro. If your going to be in a noisy enviroment the go with the 280 as it shuts out the world like nobody's business.

Great point Geoffcin. Patrick, if you go with the AKG watch for the "S" in the model. AKG K240S are the "studio" version at 55ohms. Still difficult for a typical portable to drive, but I'm assuming you're looking for a pair to go with your system. AKG K240M, that's right, "M", are the monitor version and as Geoffcin said, are actually 600 ohms. These are typically used by professionals, as you know they run gear in series and such.
I own both, I have a headphone amp that I use at home and with my portable, however the 240's stay at home.