If you're at a mate's place, you can open the boot... Otherwise follow the other instructions that I gave previously
Lower frequency bass travels further, so maybe it's the subs, the box they're in or even the music you're playing. I know that certain songs with low bass travel alot further. With my old car, I did a test at a recreational where there was no one else and played different music to see how far away it could be heard... Even did it with windows and things opened/closed. Even closed, it might not sound that loud next to the car, it still travels a fair distance.
Where is the box located? What direction is it facing? In my cars it's pretty much always been behind the seats facing rearwards and it has been loud. Facing up towards the bootlid was loud too, but you need to fix the boot lid rattles.