As already mentioned.

The beoport doesn't have a internal DAC. Therefore the quality depends on the attached source, the soundcard of the pc.

It can be improved to install a better soundcard in the pc or to invest in an external DAC (like the DACmagic).

Another point is that the quality of the output on a pc/mac depends on the attached device. The internal impedance of a headphone differs from a input from the beoport. That can make the difference in sound.

So a special soundcard with special line -out, will give a better result than a iPhone/Pad or other source wich is designed for headphones.

Of course, the shorter the route, the better the quality might be.

I assume that the used switching technology on the beoport board should be on the same level as used on the av-boards in all the Beovisions.  

Airtunes is not really comparable to Beoport since its actually a streaming solution that uses the Remote Audio Output Protocol and is infinately more sophisticated, carrying more data and content types - including 1080P video streaming. 

Sound quality will always be potentially greater via Airtunes as its carrying digital data. The ideal solution is to have Wifi and the protocol directly embedded.  Lets hope B&O's new found interest in working with Apple also picks this one up for future products. Whilst the Airport Express isn't high-end... the newer 802.11n with the Marvel chipset actually has a Cirrus Logic 4344 24-bit/192-kHz DAC embedded from what I've read - similar to that found in the Roku box etc. In other words this is extremely good, but not £££ audiophile. 

? maybe a faulty J-Box/ML-Plug, ML-Cable ?
ML is an analogue balanced/symetrical signal - if one of the 2 leads is missing/faulty the result will be poor sound.   

Heribert wrote:

What is the problem with my setup? Why is the sound from Beoport-ML-Beovision much thinner/poorer than directly at Beoport?

The powerlink output can be adjusted in Bass, treble and/or Loudness in the setup menu from the Beoport.

These settings doesn't affect the audio through the ML "output" from the beoport. That might be the difference you hear.