I have a subwoofer placed in a cubby hole intended for firewood. A few inches above the cubby hole is another cutout that has a few suspended shelves with display pieces (beer steins, vases, etc.). All of this structure is framed and drywalled, including the fireplace mantle and such. I built the shelving areas and the firewood cubby holes myself, and they are quite solid.
The old sub was a snug fit in the cubby. Literally pressed against the top of its hole. This would result in occasional vibration of the stuff on the shelves. There is even a small crack in the compound just above the cubby.
The old sub has given up the ghost and will be replaced.
Let's say that potential new sub "A" weights 60 pounds and will fit in the opening without touching the sides or top. Potential new sub "B" weights 100 pounds and also fits the opening with room to spare.
Will the extra mass and inertia of Sub B result in less vibration being transferred into the surrounding structure? Could this result in a meaningful reduction of NVH from the shelves? Or will the spl from a modern sub that's NOT 15 years old cause all that stuff to shake anyway?
FWIW, the listening area is an open concept deal that's about 9000 ft^3, plus there's a staircase open to a loft, and the back of the listening area has three large wall cutouts open to a sunroom. There is all kinds of volume and several places for sound waves to get lost. Yeah, also contemplating a sub on each side of the fireplace. Thanks!