More shipping damage again?

Shoe...while I cannot see what you did and what Fedex did; there is an outside chance that your reinforcement of the subwoofer box may have contributed to the damage.

Those factory shipping cartons are designed to ship the item properly. If you filled in the gaps in the box with more packing, then there was less space in the box to absorb the shock. The shock is then transmitted to the gear through the additional packing.

Put another way.....Large air spaces in the boxes between the packing inserts allow for the outside of the box to be pushed around without touching the gear. Filling in those spaces can cause the blow to then dent the gear by way of the additional packing.

It is a tough call. I like to add to packing myself sometimes.

Multiple boxes alone won't protect speakers. If you want to ship heavy things (speakers, amps, etc.) with confidence, use (at least) a double box and strap it (or them) to a wooden pallet. If it's really big (think $120,000 speakers) you'll need custom crates. Getting things there in one piece isn't that chancy, short of the plane, boat, or truck crashing! You just need to pay enough to do it right

All the original manufacturers packing is solid Styrofoam, not peanuts, soft foam etc, wonder why. The solid stuff is available at Home deport in large sheets. Best to double box and double line the sides, top and bottom of the space between the boxes. As mentioned in other posts, any air space will provide the ability to shift, leading to damage. All carriers are about equal, it is the packer that makes the difference. Want special handling, insure the item for $5,000. That way it goes through a better handling path. The added insurance cost is well worth it on fragile item. But in the end, pick up works best. You know exactly what you are getting.

I also recommend a wood pallet for expensive speakers even if they are in the original boxes. My Aerial LR5's came from the West coast to NYC in great shape as they were wrapped on a wood pallet.

I was looking at a couple of pictures the buyer sent to me.
If I were to take a sledge hammer and hit the top corners this kind of dammage could happen I suppose. Unbelievable.