Resmed S8 vs S9 Issues ??

Hi,
Has anyone had an issue like this?
I have used a Resmed S8 elite for many years (pressure 10). I had an opportunity to upgrade about a year ago to the S9 elite, so I took it. I had it replaced once because the original was making a lot of noise. The new one was slightly better. Since getting the S9 I have had lots of issues. Mouth breathing, exhaustion, collapsing airway, noise from the machine. I have played around with the EPR and other things as well as full 2 styles of full face masks. I had a CT scan (no issues), and repeat sleep study. My pressure was raised to 16 (yikes), but now I mouth breath more than ever, and the full face masks leak horribly.
So out of despiration I unpacked my old S8. Set it for 14, and.....NO ISSUES. I feel great. No mouth breathing (wife says so), and no noise. What's up with that? Could the S9 have been defective? Does it work differently than the S8? Has anyone else had this issue?

Thanks
Ken

As long as the S9 is under warranty; and you have a back-up, good idea to have them check it out.