New cable works
After inspecting both cables I noticed the old one was missing 1 pin in both ends. I had taken them out to improve SQ.. worked well with my old device, but not with these. Dunno why that is, maybe they need more bandwidth, but the missing pin should be there to give power, not needed with any of these 3.
Mytek should have came with cable, but both devices had lost boxes and almost everything else. Full of scratches too. I don't recommend that shop to anyone. Other was also missing power cable. Crazy.
Some things I noticed so far (more for myself), I haven't tested with speakers at all
-HDV 820 alone sounds better than Mytek (even with low impedance phones Denon AH-D7200). Larger soundstage, meatier bass, more powerful sound. So Mytek is a bit disappointing. It's PSU could be upgraded to external, but I doubt it can catch up
-Mytek as a DAC (rated better than RME by some, if that says anything) + Sennheiser as an amp sounds incredible to me, to be honest I haven't compared back and forth like I did Mytek and Sennheiser many times, because it's too much work, but I'm sure it sounds best I have heard
-I compared this combined setup to old Asus back and forth, and the improvement is quite massive. Like the other sounds ok/good, but other makes me emotional. This is not only because it sounds better, it sounds much more powerful and clean
Will probably return both, because there is no point in keeping both, and get separate DAC+AMP for cheaper. Also partly because of my neighbours, and the stress playing any louder than quiet causes me, and the sound quality I get, and the things I can hear, I plan to sell all my speakers if possible and never invest in speakers again. At least as long as I live here. Luckily I only bought 2 speakers now.