I don’t have the OPN S, only the original OPN 1. I’ve experienced the scratchy sound myself, although I don’t really know what you describe as “scratchy” is the same as what I describe as “scratchy” or not. In my case, “scratchy” means that I can hear distortion at the end of many sounds, like the receiver is overdriven, even though it’s only as normal volume. It happened on my 85dB receiver, and replacing the offending receiver with a new one seemed to fix the issue. But since you said you already replaced the receiver with a higher power one and it still does it, I’m not sure if we’re talking about the same scratchiness or not.
Do you have Speech Rescue turned on? I’m guessing not. It may be something you may want to experiment with by turning it on in one of your 4 programs and set the destination configuration to the 2.4 configuration setting and turn off the High Frequency Band setting just to see if the scratching may be due to the over amplifying on the high end instead of the low end. If turning on Speech Rescue removes the scratching then at least it’s confirmed that it’s overamplification on the high end. If it doesn’t help, you can just ignore using that spare program until your next visit with your audi to have it removed.
As for updating the firmware 8 making things quieter, I’m wondering if somehow it puts you in the OpenSound Booster mode which is more aggressive Full Directional setting than normal. Check the ON app to see if it’s activated by accident or something.
It sounds like maybe your previously fine-tuned settings didn’t carry over to the hearing aids properly after the new firmware update. Also confirm that the fitting rationale settings are the same after the new firmware update, just in case.
If none of this helps, I agree that getting the hearing aids replaced with brand new ones would be the next step.