Hey Andre,
I got the same Issue and was on 703 and now tried 704 for ~1 week. at first it "seemed" to work, but if I unlock my phone and Poweramp opens directly it shows me the "Cant use Hi-Res Output, reverting to standart" - popup.
So both versions dont work atm
Also Oneplus 3 with Nougat (OxygenOS 4.0.2)