I had email contact with a Marian engineer, who tried to replicate my problem. His results were a lot better than mine. With 19 and 20 kHz tones at -13dBFS, I saw a highest -78dB peak at 18kHz. In the data he sent me, having the same tones at -13dB, the highest other peak was at 1kHz, and at -116dB.
I didn't have it replaced - unfortunately it was secondhand, and I didn't have any buyers receipt.
Edit: what's particularly funny about this thing is, that according to the guy at Marian, all data goes bitperfect to the DAC. Even volume adjustment is done at the DAC. So, if any intermodulation occurs, it's happening in the analog section (or in the DAC).
Now
here's a test report of Hoontechs DSP24 Mk2. Same AK4524VF DAC, same IMD pattern as I had! However, Egosys' WT2496 also has this AK4524VF, and its
test results show exactly the same pattern as the Marian engineer sent me: only a peak at 1kHz...
Either this AK4524VF had bad and good revisions, or it is very sensitive to the quality of the surrounding components... ?