Pro and Cons of Firing a sub upwards?

I am looking into building a sub box and wanted to know what the best position for aiming the sub would be. I was going over the ideal of having the box against the back seat with the sub pointed upwards at the rear deck. I have an area on my rear deck where a factory 10" is now located. My intention would be to remove that sub and put the new one directly under the opening. A facade would be in place to separate that area from the rest of the trunk.

I have been told by a couple of local shops that if I place a sub box in my trunk against the back seat with the sub facing up, a 3+ db gain would occur.

What do you guys think? Would it be better to just point the sub towards the rear of the trunk or maybe towards the back seat?

the reason it would have a 3db increase is because the sound wave has more room to bounce and will cause a bigger sound wave giving you more sound pressure. if sound pressures (SPL) is what you want then you will want to fire it up

but if you want sq sound quality then go with the sub facing the back of the seat