Re: 5.1.4 Atmos System
Look at this thread, post #32 by member N. Boros. This is an excellent way to figure out how much sub you need. It really comes down to how big a space you are playing into (cuft is more important than sqft, and BTW, 28x15=420, not 378, but that might be throwing off some of your calculations), and how loud you need the subs to play in that space. And you're the only one who can answer those questions. And for the record, I've got a pair of LVX12s that I love, but in a smaller sealed room (about 13.5 x 21 x 8, about 2250 cuft), and I mostly listen to HT at -20dB below reference, and no higher than -10 dB, so I don't need gobs of power from my subs.
For anyone who is building new space for HT and/or music, I typically recommend some level of acoustical consulting. Making a room sound nice is difficult and not at all obvious. I suspect that most consultants would look askance at your dimensions (28 is almost 2x15) and all that wall area covered in glass (doors/windows) for example. Also, construction details matter, especially if you're trying to sound proof (that is, keeping outside sounds from getting into the room, and HT sounds from getting outside the room). This will really matter if you like to play HT/music after young kids have gone to bed, or if you share a wall with a kitchen that includes a noisy appliance like a Vitamix, or if you share a wall/floor/ceiling with a laundry room (washing machines can produce a huge amount of midbass noise). For what it's worth, my problem is neighbors and their lawn crews using power tools in their yards (the howl of gasoline powered blowers can penetrate just about anything). So it doesn't have to be an attached wall.
Anyway, good luck with it, and enjoy your new room!
"If it sounds good, it is good." -- Duke Ellington