I also have a BB exhaust.
One thing I did last week was to line my entire trunk area, including some of the area behind the fenders, with inexpensive sound deadening material, then placed a moving blanket in the wheel well. I can tell it softened some of the low frequency sounds in the cabin.
KILMAT 80 mil 18 sqft Car Sound...
https://www.amazon.com/dp/B082Q2J2T4?re ... b_ap_share