Sorry to break the in-ear headphone only rule but I figured since we use the same phone and amp I could help out. (I switch between cheap sony earbuds for portability and convenience and Sennheiser HD-555's for when I'm not going anywhere for a while)

Sound on the N900 is very average. Don't let that fool you though, there has to be a standard set somewhere. The N900 is very listenable, but no more listenable than say, an iPhone. I find my G1 to have pretty good sound and my Cowon S9 to have excellent sound. The N900 is perfectly fine if you're walking/chilling between classes but won't be replacing any reputable media players anytime soon.