I agree with both possibilities here... I will say that I have dual Plus/2's in my 8000 + cu ft room, which opens to everywhere and I find there is a lacking of impact in comparison to my 1700^3 bedroom with a single JL F112 in it...
I would suggest that your single Plus/2 in that huge room isn't enough, but the T shape might aid in negating the usual standing waves from a standard square or rectangle shaped room....
As big as the Plus/2 is and as much bass it puts out there are limits to how much space it can pressurize. 4 subs in that room would be great, and you may want to think about adding more. I will certainly be doing that for my gynormous room.