ROCK on NUC8i5BEK/8GB/120GB SSD for database + with music on USB disk.
End point 1 is dedicated Pentium N3700/Win10Pro/PinkFaun SPDIF Bridge/Win 10 Pro.
End point 2 is dedicated Celeron 847 with on-board SPDIF/Win 8.1.
All machines up to date software-wise.
The sound quality on end point 1 decreases when I play music on both end points. ROCK does some DSP for both end points: a -2dB dip at 3000 Hz with Q=1. Library is roughly 20,000 local tracks + 2.000 on Qobuz.
Thanks for the tip! Only now did I find out that each end point was actually running a 6 band Graphic Equalizer. I did not realize that Roon stored things that I tried in the past in different bands. All of them had zero dB gain, except Band 1 (see above) and a sharp low pass filter above 20 kHz (which I cannot hear anymore anyway…). Problem solved now.
Sorry to have bothered you with such a stupid mistake…