thanks a lot for your response.
Yes there's an issues with HDMI. The AUdio is only an embedded part of the whole signal that have do be encoded and decoded. Moreover the signal runs (depending on the Receiver) through several Video processors. And yes I tested with my equipment and the difference to the (electrical) SPDIF is definitly there. However my receiver only accepts the 96Khz and I have some tracks also in higher res.