Any ideas as what is causing the volume difference?Hi @elf1
Regardless of what's causing the volume difference, in all likelihood you don't have any problem or issue with your gear. IMO, 9 is fine, despite what some proponents of 10 or 11 will claim. Also, keep in mind that irrespective of your CD player, OPPO, or other source, the "level" of the program music source itself also plays a major role, perhaps the most important role.