I've spent a coupla days with my new Arrow 4G. Perhaps I have tin ears... with all eq and effects zeroed, I hear no difference with the amp inline. This is out of an iPod shuffle and a Macbook Pro, and into AKG 702's and Rain 9mm earphones. Listening to old and new recordings of acoustic jazz at a variety of volume levels. That's not to say things don't sound great -- just that they already sounded very clean and detailed without the amp, and the amp added no coloration (good or bad)
Meanwhile, the coloration you can add with switch settings is generally subtle and agreeable:
The bass boost is smooth and wide, and is a nice addition for both headphones, especially at low volume levels.
The treble boost is a nice addition for the 702's, and made a nylon-stringed guitar sparkle in just the right way.
The crossfeed is very subtle, but welcome on my mid-century jazz recordings where instruments tend to be hard-panned.
The combination of switched gain settings and a volume knob is very convenient.
Output to two headphones at once sounded great, and is a really nice feature.
I'm still debating whether to hang onto this thing. It is a very attractive, very cool device, no question. Maybe I needed hungrier phones to hear a bigger difference.