I borrowed my roommates nice earphones to compare them to the ones which came with the phone. I used to fancy myself a pretty decent audiophile but perhaps in my old age, I just didn't hear THAT much of a difference. The bass was definitely deeper but I actually preferred the high-end on the Nokia earphones. That being said, I am pretty picky still and I was VERY surprised with how nice supplied earphones were.

As for the audio quality I have never been able to afford a proper setup as you mention but that aside, the N900 is probably the best sounding phone I've heard. The audio player was lacking an equalizer for me to make fine adjustments and I was worried about that, but after listening the output I didn't really feel that it needed it at all. As this is a very subjective area your mileage may vary but I walked away pretty impressed.