Axel - I am assuming your Mutec has shipped with the new V1.10 firmware. (I know teddlesdee's Mutec had the latest firmware, and his was shipped a couple of weeks ago) You have to select 'Intern' and 'Re-clk' plus the USB input as the 'Reference' (second column of lead's). Explaining how exactly is more difficult than actually doing it. There's only two selection /input buttons on the device and pressing them will show you what they do. The first column of lights shows a green light on 'Intern' and 're-clock'. The next column of lights should be green spanning one or both of the USB-PCM / USB-DSD. On the latest firmware the top three lights glow with a PCM input. (only two green LED on the previous firmware if I recall correctly) The next column 'clock out', all light blank. The 'clock multipliers' should all be top row 'x1'. You should then have the blue 'lock' light and the red 'Audio' light. I would post a picture, but I'm miles away from my Mutec at the moment!
1000 Pro - KEF Blade - iFi Zen Stream - Mutec REF10 - MC3+USB - Pro-Ject Signature 12
