I feel for you. I went through the same exact thing if you haven’t seen it:
https://forum.psaudio.com/t/bhk-pre-amp-and-speaker-hiss/9838
The most comical part was my dealer sold me the BHK Pre to stop the hiss in my DSD Sr. I ended up moving on to a McIntosh C1100 (and more recently a Levinson No 52.), could never find a solution to make the BHK Pre work in my system without the hiss.