Hello, I recently purchased a sound level meter to check my speaker levels. My speaker manual says to set the scale to 70 db and set all speakers to 75 db. This caused me to dial my speakers back -3.5. My question is why 75 db and not 80 db for example? Is there a reason 75 db is used? Also, should I attempt to set my sub at the same 75 db level? How do you set your sub level?