Ported sub box position

True

Hearsay & speculation. It's not gonna make a huge difference as it's a big boot, but it's all to do with phase cancellation and reflected sound. Ideally you want any reflected sound waves and direct wave to hit the driver roughly at the same time otherwise you get phase cancellation which destroys the thump. So by facing the sub backwards most of the waves will bounce off the boot and into the car. With it facing upwards, you'll get a mix of waves bouncing off the underside of the parcel shelf and others bouncing off the boot, so the sound hitting the driver will be a right old mess.