Sennheisers have come down a LOT in price the last few years. I have a pair of HD212Pro's (maybe, they may have gotten torn up by now) that are my "backup/reference" pair. I call them my reference pair because I've owned them for 6 years and my ears got REALLY used to thier flat response. I've not had them plugged in to my player because a year or so ago after getting sick of thier shoddy cables breaking, i rewired the things to have RCA plugs on the end...and I don't have my minijack to rca adapter anymore.
I own a pair of Bose Triports...the over-the-ear ones...they're nice headphones too...and when I came to my first conclusion about the quality of my chi-pod...it's what I based 'em off of (again, because I use them every day).
But, earlier I took and hooked my mp5's output in to my sound card and recorded a sample or two..aside from some filtering on the analog side, I could not see a huge difference in the output.
audiophiles are a fickle and weird bunch...and I would know becuase I'm just another banana amongst them. most of them wouldn't be caught dead with an iPod anyway because "digital audio is evil". I put a bunch of .flac audio samples up at
Index of /player (forum automatically converts to link and changes to title, but there's no page) - one is the original (1000000.flac) and the one suffixed 2706 is off my player. My post titled "what player is this" has the details on how the process was done, but rest assured the audio was converted properly before and after the process to eliminate conversion variables, however, ultimately, the quality of that file will depend on the quality of your sound card, so to get a better idea...play it in a better sound system.