This might be worth trying; after iPhone wireless/Bluetooth connection to your car and before turning up volume on your infotainment system, open your phone menus to examine sound settings. I'm presuming Bluetooth doesn't have a volume adjustment so check sound settings and when you open the page to sound settings, raise the volume and see if your Subaru ramps up volume at the same time. There's another setting to examine but in your infotainment system's sound settings.

Open your infotainment to sound settings and examine presets. There may be a default setting and you can raise initial volume settings to a value. After leaving that page, go back to whatever you're listening to from your phone Bluetooth connection and see if the new setting is satisfactory. You can always readjust infotainment sound settings to your liking if this works.

 

I will give these a try. Thanks!