When you utilize bass management, the bass is re-routed to the subwoofer. So while the lfe channel is spec'd up to 115, the subwoofer can get more adding in the other channels. For example the movie Skyfall expects your bass-managed system to put out a continuous 121.4dB at one point if you are listening at reference.
Personally in your space I would go with dual F25's or the subm m/s pair. If you set them up both up front, just use a y-splitter and gain match if F25's. Even just on each side of the center laying down, you should be at 25% room width and therefore cancel those modes. Bass will be a bit smoother and non-directional.
If you are spending this kind of $, whats a hundred bucks+ for measuring gear and a some time spent to get the most out of your system? TIme domain and frequency response are proven to have high correlation with sound quality, and our ears are terrible measurement devices. Then if you want to move a sub to the back for possible better results, add delay, eq... you are prepared.
If I had to go with one, I would get the Triax at its current deal. Tough to beat, but adding another one later may be too much $, and I just can't settle with a single sub myself.
You can't wrong with the sub choices you have been given. Going from a single to 3-4 15" drivers will be worth the change IMO. Good luck!