When I first got my Cowon S9 and started using it I thought, good enough player, above average. Then I updated the firmware and it's like a different machine to me now, has so many more features it never had before.
The sound quality to me is fine, up to par with anything else. I use the BBE Viva 2 setting, widens the sound abit. I can't complain about the S9 at all though, massive battery life, I'd recommend it. The firmware it shipped with though was lacking big time. Luckily it only takes a minute to upgrade.
My IE8's probably have alot to do with the great sound though, massive bass, everything clear and crispy.
I wonder if the sound improvement should owe to IE8s? You see, IE8 is more expensive than S9!!!