I bought my pair when they first came out so have one of earlier ones. They do sound different from a pair I heard that was bought about 6-7 months later, with the later pair having more treble emphasis, and a bit less on the mid-bass (iirc...it's been awhile). Both sounded good, just a tad different. Neither had too much bass imo, and I'm not a basshead by any stretch. I think slwiser didn't like the bass of the earlier ones, but NightWoundsTime has a pair, and I think he's pretty pleased, and Dan's been using mine for a bit now, and might be able to comment.
If you're going to use standards that compare them to excellent full-sized headphones and rigs, they are going to fall short. Yet, they are really enjoyable, excellent sounding closed headphones made for portability. I really like you can get a hard case (just bought another), where the pico fits perfectly in a pocket, with another pocket available for the usb cable or lod, and you can pop in an ipod there as well and have a compact, take anywhere great sounding rig.
Got this pic off google search. It's not mine.