Zen Micro
Nomad Jukebox 3
Rio Karma
Iriver H-140
The Rio Karma is hands down is the best sounding player. It sounds marvelous with my shure e2c's, i can't praise the synergy enough. It also sounds great with my 7506's and it even drives my Grado SR100's ruthlessly (they sound stunning)...again, the karma has great synergy thanks to its great eq.
The Zen Micro sounded lifeless with my shures, it sounds pretty good with my Grado SR60's and 7506's.
The NJB3 is pretty close to the Karma in terms of power (volume) and being able to drive the SR100's effortlessly, even moreso than the Karma (i only need to have the volume set to 15 compared to 25 on the Karma). They also sound very good with the njb3 in terms of enjoyment.
I just purchased the iriver, so i'm still learning to use it, but the "jazz" eq sounds pretty darn good with my shures so far. I can't say much else as i haven't tested it as much as i would like.
calvinhobs, i imagine you still haven't decided on a player. I'll say this much, based on personal experiences...stay away from the Karma, mine has had issues with it: repeated freeze ups when shutting down, jog dial broke off and somehow screwed up the power on/off workings of the player (sometimes i can use the red nipple to turn the player on!). Regardless, stay away from the zen micro, i didn't think much of its sound quality, its too wimpy in terms of loudness and the ability to give that impact you expect from many headphones, like the 7506's. The Iriver was/is very expensive to purchase, so scratch that off, unless you got the money. The NJB3 is no longer in production, so you'll have to secure a used one, it's built very well, but hell, its big and heavy.
My opinion? Take a look at cowon...many folks seem to dig their players and i would've picked one up a few weeks ago if they were sold locally.