I had the same issue. To get it into DFU mode you have to turn the device on, connect it to your laptop and then hold three buttons at the same time for five seconds: The Plus button, the Minus button, and the Bluetooth pairing button on the side. Once you've done that then you can run the DFUWizard software using the instructions provided:
Instructions:
3. Find and open the program "DFUWizard".
4. Click "Next",
5. Select "USB" and click "Next",
6. Select option 1 or option 2 and click "Next",
7. Click "Browse", find latest .dfu version file, click "Select", and click "Next".
8. Click "Next" to begin upgrade.
9. After completion, click "Finish".
10. Enjoy!
Unfortunately, it did nothing for the noise floor issue. It's still a white sound machine at low volumes.
Christian at Plussound has been incredibly responsive in trying to help get this sorted and gets an A+ for customer service. Unfortunately nothing so far has helped. He's asked for them back to evaluate, and hoping he can help.
I'm listening to them on my JH16v2s and for anything with pauses or at volumes at 30% or below it's like listening with a white noise machine in the background. And forget about classical music. Just for fun, I listened to the music in this article:
https://www.nytimes.com/2019/10/02/arts/music/silence-classical-music.html
It was basically intolerable to listen to the pieces referenced
To make sure it wasn't the headphones but the plussound device, I went back to wired and blissful silence. For fun I also connected to cables to my Radsone ES100 and again, blissful silence.