I'm trying to calibrate the sound level of my sub, but I'm noticing that the SPL meter isn't giving a consistent reading, even at Slow response. It "jumps" about 2 Decibels back and forth, sometimes even -3 / +3 Db's. Is this normal? I'm using test-tones generated by my 3802. Also, what would be the best volume setting for the subwoofer output of the denon?