yes... imagine your average subwoofer box travelling at ~60mph hitting the
back of your seats... all that is required to remove most back seats is
light hand force...

2? maybe not... did i suggest using only 2? though two would be better than
bungie cords any day i'd think...

i just read a story about a girl catching a box with her head... she won't
be breathing again anytime soon...

http://www.teamrocs.com/news/modules.php?name=News&file=article&sid=19

and i know a guy... a fellow teamROCS member who had a box shift during
cornering causing him to lose control and, if i remember correctly, total a
perfectly good asian shitbox...
--
sancho
it's your a.s... do what you like...

Unless you flip over a few times. I'm gonna fasten mine well in my hatchback
after giving it some thought. Think about it.  You flip the car over, the
box comes out.  Flip it over sideways after that and it's in your head. I'm
not gonna risk it.

don't come crying to me

It will, easily. I'll bet that unless you're a weedy little runt, there's a
good chance you'd be able to lie in the back of the car and break the seat
backs just by kicking hard against them with your legs.

Now, go here and read:

http://www.vcu.edu/cppweb/tstc/kinetic.html

They point out that forces of 40 to 50 G are routinely recorded during
the standard 30 MPH crash tests.
Your sub box weighs, for example, 50 pounds.
Multiply that by 50: 50 x 50 = 2500 pounds.

That's well over a ton. If you were to suspend your car nose down, then
stack over a ton of weights onto the back of the seat, do you think the seat
locks would hold? Would you be prepared to be buckled into the driver's
seat while the weights were stacked? What do you think would happen to you
if more than a ton of weights landed on you? It'd be ironic if you were to
hit something, be saved by the seat belt and airbag, then be mashed to
strawberry jam by your sub box.

Bolt that sub box to the vehicle frame. NOW.

It needs to be bolted down as securely as a trailer hitch would be mounted.
This means high-tensile bolts and big load spreading washers. The bolts
should run right through the box- it's no use just bolting the bottom down,
the rest of the box will likely rip off it and go flying.

If you doubt that, just take a sledge hammer to an old sub box sometime.(*)
I'll wager that it'll only take one big swing to break out the bottom or a
side if you hit it from the inside near one edge (puts the joint under
maximum tension). The sledge head weighs 20 pounds or less and is moving
less than 30 MPH. I'll bet your sub drivers weigh 20 pounds or more, and
they will be moving faster in even a moderate collison.

(*) Yes, I have done this. They were water and smoke damaged and had to be
written off. We broke them up so they'd pack flat in the dumpster.