Which reference level? THX reference level is 85db for the speakers and 95 db for the sub with 20db of headroom for each! Most manufacturers(incl. Yamaha) refer to 75 db with zero headroom as reference level because it's easier to attain. Theoretically if you ran pink noise through each speaker individually and set each speaker level to 75db with a spl meter and set your volume control to "0",you would have your 75 db reference level. Of course,that will be really loud,possibly damaging your speakers and/or receiver,a level you probably wouldn't listen to for long. Now,if you turned the volume down to -30 would you be hearing 45db? Unlikely due to different calibrations of volume controls. Regardless,YPAO will not calibrate your system to "75db reference level" at 0 volume level. Think about those THX reference levels and you can see why stuff that says THX Ultra 2,Select 2 or whatever costs so much and why most people don't need it.