One or two subs

WITHOUT any doubt...2 subs are better than 1. I believe it is best to have 2 of the same subs which ever you choose. Get the best subs you can afford. I use a single Rel in my 2 channel - using the Neutrik Connection. But it is barely on...the crossover is set at 25 htz and the volume is almost off. It just ever so slightly underpins my mains.
BUT<<< in the Theater I use 2 subs...(won't mention the brand)....In theater the subs play a bigger roll. They will probably be crossed over around 80 Htz and be considerable louder. Here's what I find...Having 2 subs eliminates any localization of the low frequencies as well as improving upon room modes - Much smoother response. The bass comes at you like a wave. Ever once in a while, only 1 sub will kick on and the room seems lopsided. then the other kicks on and it's like both feet planted on the ground.
Not to be a party pooper but the rel will run out of gas on some content dished out in Movies - Try the wooshe finger hold at the end of Kung Fu Panda 1- that is a great torture test...Good Luck and happy listening

For the members who have two subs, how is it positioned?

For my 2 channel I have 2 F-113 subs. I believe it sounds better with 2 over 1.
But for my Home theater I have 5 subs.

Left & Right Front corners...

With my 2 channel F-113's. I have those subs outside of my main speakers near the left/right corners. But, they are out into the room a couple feet both at the back wall and the sidewall. This will limit bass boom.