I have answer this very text in someone's question, but I am going to go ahead and post my own question, I hope someone from presonus reads all this:
Hello, I have a couple of S8 since a couple of months ago and I am experiencing some issues too.
1st: both of them had a hiss when no sound being reproduced which wasn't exactly annoying but none of my previous monitors where having. (decided to keep going because they sounded very good)
2nd: 1 of them has developed a noise, like some type of cracking background, its not caused by the trs cable, it does it even unplugged, it is also not an electric issue, as I have a good setup.
3rd: the same monitor as above, the one with the cracks, suddenly after turning on, the volume wasn't matching the other monitor. Had to turn up gain quite a bit to match the other monitor on 0 gain.
Trying this: https://forums.presonus.com/viewtopic.php?f=77&t=22696&sid=cf85c97015c8eb943e4aee48bc1024a3 resolved the issue of the volume but it immediately sounded dull/boxy, so I rebooted with all 3 DSP buttons clicked and voila! volume and good sound again (however the cracks still there).
4th: the other monitor which didn't have the cracking, trying the DSP reset, one of the buttons literally broke, its not acting no more. Stuck on linear. BUT, if you click other DSP buttons, it may switch (randomly) weird ah?
I am about to send them back, I don't know if I am unlucky or this monitors have been crafted with really cheap materials or very bad quality controls. Although the sound is very good, which is a pity.
Hope it helps, and if anyone from presonus is reading, guys your flagship monitors are trouble...