It's OK to ask once.
All I have to go on here are the reviews Shady did. The Monolith has a bit more output than the X13s. Nothing wrong with that. When I first set my subs up in my ~2000'3 room, I did the ol' 'set to 50%' thing and started playing music. Within 5 minutes my ears were going numb! I started this slow process of turning them down, little by little until that feeling went away. Right now, when I run 'em hot, they are only abut 1/3 gain, and trimmed down pretty significantly in the AVR.
The nice thing about this is that I am never taxing them, really: never driving them to the edge of distortion. In that regard, I know my bass will always be very linear and clean, and I have a lot of headroom to play with if I choose.
So, will one Mono 15" take care of your room? Yes. I think the 12" will too. Would I still recommend two, yes. Would I encourage you to look at Hsu, instead? Honestly, I can't say either way. Both have good reviews. Hsu has a longer reputation. Both are in LA and ship from there.
I will say this about the Hsu subs: Both the VTF3 and VTF15 use the same driver and amp. the only difference is the size and tuning of the enclosure, with the VTF15 being larger and tuned ever so slightly lower.
Beyond all of this, you are really coming down to a discussion of budget and features+ergonomics. Only you can answer for yourself which sub will be right for you and your system.
Fun times, the decision of what sub to buy!