I think setting the sub 15dB less than the rest of the speakers is incorrect. This would result in a sub level that is much too quiet.
Refer to the calibration help section on the
SVS website. They say:
"As the tones start, alternating, speaker to speaker (watching your sound meter now) set each speaker’s volume to 75 dB, using the receiver’s channel controls....What to set the sub to? You might find that a
higher level, relative to your main speakers is preferable. In other words, set to a bit higher than 75dB..."