You were running 15 on fixed pressure CPAP with an S8.
Now, you need to bracket that number when using the S9 Autoset in auto mode.
Suggestion: go to 3 below and 3 above your Rx pressure. Such that min pressure is 12 and max is 18.
You don't want to start at 8 -- it's too far below the Rx pressure.
Plus to take advantage of auto mode, you need to give the machine some working headroom.
Download sleepyhead or resscan so you can follow your progress.
If you keep banging up against 18 most nights, you might have to raise max pressure.
20 is the most the S9 Autoset can be set to.
(You could set max to 20 -- the machine should not use more pressure than is required.)
With time, you may be able to narrow the pressure window.
When you have congestion, you will generally find you need more pressure.
Tuning the humidifier to get optimum humidity may ease congestion.