Guys,
The IMP-400 IS worth the difference over the IMP-350's price, IMO.
I bought the IMP-350 for my wife last Christmas and just received the IMP-400 for myself
Using Etymotic 4S's, the IMP-400 has a fuller, richer, yet very clean and clear sound compared to the 350. The FM reception also seems to be much improved--with the 350 it is often difficult to get stereo reception; with the 400 my experience so far has been much better. I know IRIVER's website doesn't seem to indicate any differences in specifications between the two units, but that is my own experience (I have been listening to both to compare). The remote is also a bit nicer with a SLIGHTLY larger and brighter lcd screen.
Finally, the 400 comes with 1450 ma NMH batteries, as opposed to the 1400 ma batteries with the 350. And the 400 has a very USABLE and cool case which has a belt loop attachment (different from the pouch in the picture posted above), plus a car kit (12v adapter plus cassette adapter to allow you to charge and play your unit through the car's stereo).
The headphone out of the 400 is the best I have heard since my original Sony D5 player. It's THAT good (Okay, I don't have a collection of old Sony units, so don't flame me--but I did own a D5 and it was GREAT sounding--but no skip protection, barely portable, etc.)
I have yet to listen to the line out (I intend to use my Sony D-EJ1000 for that purpose, together with my Cosmic), so I can't report on that yet--but those of you looking for an excellent headphone out--look no further in an MP3/CD player. This one has the goods.
One final note: when my unit arrived from J&R Audio, the firmware was corrupted/DOA. I attempted a firmware upgrade, but was unable to get it to initiate completely until I hooked up the unit with my wife's 350 remote. Once the firmware intitiated, I was then able to use the 400's remote. I'm not sure why this was. Perhaps there might have been a better way to do it and get the same results--but the 400's remote was unusable (no characters visible past the opening screen) until I plugged in the 350 remote. Go figure
I'm sure some of you out there probably know what I did wrong/should have done differently.
At any rate, I'm guessing the transit of the unit caused the firmware to be corrupted, but as it turns out, all that was needed was a fresh loading of the 1.02 firmware. Just a friendly warning, because based on my review (if you believe me
, ) I know alot of you are going to be ordering one soon