Agreed with the buzzing/vibration comment.

I recently installed the W series and yes I believe the reason for the spacing is to allow the bracket to work. To make sure you don't have any buzzing issues, why not surround the box with a layer of insulation (pink or Safe n Sound)?

Looking forward to reading your impression of the new setup!

Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1