This quote is from Audyssey Zendesk . I about fell off my couch when I saw this quote. He also says that your sub volume should be turned up to 50% and should only be turned down if you get a -12 level on your sub. He suggest keeping your volume on your sub the same, even if you get a -11db level on the sub. So, is the +/-3db subwoofer level a myth?
Just a bit of friendly advice. When you reference something like you did in your post, it is a common courtesy to include somthing in the post that points the reader to the area in the quote you are referring to. The Zendesk thread is very long, and you force readers to search for the response you are concerned with. For example, you could have said, look for Chris' response dated July 1, 2011.
IIUC, this is the response that concerns you:
"July 1, 2011
The setting of the sub volume knob doesn't matter as long as MultEQ doesn't come up with a trim setting of –12 dB for the AVR. If that happens then you are at the limit of the AVR adjustment and you don't know if more level cut is needed to bring the sub level to the required reference setting. Any other trim level setting is fine. Once you set the volume control on the sub it's best not to touch it. If you want to make personal preference level adjustments to the sub level then you can make these in the AVR menu."
This has been extensively discussed in this thread already.