The RS-1s are rated at 32 ohms and are easy to drive. Many headphone fanatics feel you can use them with a portable player like an iPod without a problem, but will benefit using a headphone amplifier in conjunction with the portable player. The Sennheiser 600s and 650s are rated at 300 ohms and will sound best used with an amplifier; I doubt you'll get sufficient volume plugging straight into an iPod with the 600 or 650 models.

It depends on what you call high end phones. If you are opposed to a headphone amp, then I suspect the best sound you'll get will be with Shure in-ear E3c, E4c, or E5c. They'll run between about $200 and $500, they're about 30 ohms, and you can drive them straight off the iPod. The sound is pretty good with that setup - assuming the source format is AIFF or Apple Lossless -but it gets materially better if you put a little portable amp like the Headroom Total BitHead into the system. As for top end stuff like the 600,s or 650's - just forget it. You're talking 300ohms. You'll eat your iPod battery up in no time and, even so, will have been unable to give the phones what they need to show their stuff. Think about a portable headphone amp and good quality 30 ohm phones. Check the Headroom website. They have a useful comparison feature and a wide array of phones from many manufacturers as well as their amps. Good luck.

Take your POD to a Grado or Sennheiser dealer and try all in your price range. Buy what sounds best to you. It is that easy. I use Grado 60 and 80's with my U2. Sounds very good to me. For sealed you might try the Senn 280pros.

Jim gives a good open can reco with the Grado SR60---you basically can't go wrong at the price---unless you want to isolate yourself from the outside world in which case his mention of the 280Pro is good. But if you really want to knock down outside noise the Shure E4 and Ultimate Ears super.fi 5Pro are both great. Recommended lower cost IEMs (In-Ear-Monitors) would include the Etymotic 6i and Ultimate Ears super.fi 3 Studio. The best full-sized can that might run on an iPod would be the Sennheiser HD595, but it would be very hard not to start talking about an amp at that point.