This !!
There are calculators online that will tell you the relative mass with g acting on it uk the event of an accident.
Having a quick look a 1kg sub could become more than 200kg in the event of a 30mph accident, 200kg of unsecured weight is not only going to move the seat to find its way out, but will also remodel your legs if they are in the way.
So it’s good you’ve got it bolted in ?
I'll never forget the sound of cassette tape boxes flying around inside the car when I crashed my car at 70mph in the mid 80's, my life was saved by wearing a seat belt(not that common then) and by the fact the car was going backwards when it came to a rest, not a single body panel wasn't damaged.