USB is perfect digital. The crappy amplifier and DAC that are in the internet tablet(and most mp3 players and cell phones) can be bypassed, so you do get exactly the same quality on your n800 that you do when you plug the bithead into your desktop. All the audio processing is done on the bithead itself, all the n800 does is just send it the song in digital format.

The headphone port on the n800 is ****, like it is on most portable devices. A line out port could bypass the crappy built in amp (which is what you would use with the Airhead) but the n800 doesn't have one.

Hooking up an Airhead to the headphone port wont impvrove sound quality at all. It will just make the crappy sound that comes out of it louder. Amps are only meant to be hooked up to line out, and bypass the built in analog amps.

Oh, and iBasso D2 has been released. I want one SO bad. http://ibasso.com/ShowProduct.aspx?ID=38