I have a 4-door 1997 328i and I'm installing a basslink subwoofer in the trunk on Tuesday. Both of the rear seats in my car fold down and I can see the trunk if I look through the tiny gap between the two fold downs seats.

Does this mean that I don't have a sealed trunk? Or will the bass still get trapped with both of the seats upright?

You will get bass.

You can get more bass into your car if you keep the one seat down. I can tell a significant differance with my sub.