CodyRed,I had 7Bssts in a smaller room than yours 11X15,my speakers are Revel 50a's.
I don't know how they compare with Mirage,but when I auditioned them in a room at least 4X bigger than mine they sounded excellent without a sub.
But I know for a fact using a sub can dig alot deeper than box speakers especially one of high quality.
Anyway in my small room the Revels sounded great but I wanted more low end so I installed DD-15's much better low end.
In our bigger room 12X19 I use 28bsst's and I use to use on/off with the sub,I prefer the sub as it digs deep with tons of mid bass punch.
The sub is an 1812

,as I went thru DD's F-113's in here and none compare.
It really sounds like you need a sub to dig deep,I know I did as I to was using 7Bssts and now 28's.
I also think with bigger rooms the sound opens up alot more instead of being compressed with no where to go in a small room(I have had both).
The sub(s) made it for me even using my 28s in this room.
Haven't heard a box speaker that can dig deep as a high quality sub yet.
Some speakers can go pretty low but until you hear a great sub it will free up the upper end and you'll wonder how you got along without it.
My 50a's have bass but not like my sub,some prefer main speaker bottom end,I don't as it doesn't get close to authoritative bottom end as a great sub can.
Like I mentioned I to had a smaller room than yours and preferred the subs using the 7Bssts and 28's.
Hope you get her figured out as there's nothing like authoritative bottom end with mid bass punch .