We've got a PA setup for DJing events, and we use a single Behringer B1800D-Pro alongside 2 Alto Professional 15's (800 watts each). The biggest problem we have is getting even bass throughout our venues. The Behringer pounds behind the table, and in certain areas in the venue, but sounds weak in others. Your room is much bigger than what we would do use our system in, but I suspect getting the bass to be evenly distributed is going to be a challenge for you as well.
Sorry for the bad news, but acoustically, your venue is a nightmare. Square rooms create really strong resonances, and the concrete floor won't help. However, a good EQ will do a lot of good.
As for the subs, I hear that folded horn subs provide great SPL and distribution. I would recommend 4 subs (18") at about 1000W each. I have not done a venue this big before, but consider it an educated guess.
As for placement, consider placing them in two pairs at the back of the stage at thirds of the room dimension (33ft and 67ft) for acoustic coupling. This is constructive interference of the sound waves that happens when you place two identical subs next to each other. If that doesn't sound even in the room, break up the pairs and disperse the 4 evenly across the back of the stage.
At my church they place the subs down in front of the stage on the floor, and that works out very well, but it may not work with your particular needs in terms of space. It's worth a try.