When I shopped Rythmik several years ago, I had some interesting conversations with Enrico. The thing to consider is that multi-sub isn't so much about sheer output SPL, it's about smoothing the room modes that will affect the Low Frequency performance in you room. Needless to say, this can be a tricky and expensive rabbit warren to delve into without some measure of understanding...
The balance is
overall room volume against
high quality bass where you are seated. You do not need to pressurize the entire open room volume to enjoy good bass in your Listening/Viewing area, so no, you do not need to go out and buy four 21" Subwoofers that can output at 130dB each. That's the good news.
The flipside (not really bad news) is that to gain the benefit of good quality Bass in your main seats, you need a little freedom to experiment with placement. In an irregular shaped room that is not closed off and completely rectangular, the common "rules" start to break down.
One solution is to put one in the front of the room, and have another "near-field" perhaps right behind your sofa or chars, or as an end table. Another solution could be outside your seating area; perhaps on the wall between the "office" and the "gym." Likewise, on that "backwall" opposite the area in question against the "stairs." Of course, there is no guarantee that either of those spots would work.
At any rate, the kicker is getting good quality bass that sounds good across the seating area so more than one person can enjoy the "sweet spot" can be tricky, but is well worth the effort. Of course, you can be greedy and make your seat the money spot in the room, and the wife won't know what she's missing! You could put the sweet spot right in the middle, but both of you could end up getting no bass at all until you move locations... and then be like "why is so so loud all the sudden?!"
At any rate, more about raising awareness here than trying to convince you. That said, any moderate dive here on AH and you will see we tend to be strong proponents of Multi-Sub. I'm in a small room, 11x15' and about 2000cu.ft, and right now I have 2 very capable Subs that could hold their own in a much larger room. When I move around, I'm also never asking myself "where's the beef?"
Keep in mind, you can always start with one, and add a second, later, if you find it necessary.
Cheers!