Ok this thread is perfectly timed for what I am going through right now. I recently went from the Weston UM1 earphones (probably the best all around earphone for $100) to the fantastic Westone UM2 (pricey but worth it)
# Sensitivity: 119 dB/mW
# Frequency response: 20 Hz -18 kHz
# Impedance: 27 ohms
# Driver: Dual balanced armatures with a passive crossover

Anywho, I placed a bunch of FLAC files on my n810 and listened to a variety of albums the past week. Everything from Radiohead, Classical Music, Beatles, Primus , etc. Even with a lossless codec and audiophile grade earphones, the n810 always lacked low end and overall clarity of sound. I have done numerous comparisons between output from n810, ipod, ipod shuffle, and direct from pc. The ipod shuffle even sounds fuller than the n810. I am likely going to pick up a zune for my music needs since I can convert FLAC to WMA lossless easy enough and I hate apple, but that is a different thread all together.

So, I am devoid of any super technical data like most of you on this thread, but I do have some solid "ear time" with high quality earphones and the n810 and it doesn't seem like the ideal choice. Moreover, lossless audio libraries start piling on the gb's real quick and a larger hdd bases player would probably be a better choice.

PS- Anyone own a Zune 2 80gb and have some feedback? e.g. sound quality, usage, etc.