Speaker/sub for large room

I own Focal 714S towers being driven by a 50 watt NAD C325BEE in a large 31'x15' room with 7.5 foot ceilings. The room is very bright as there are mostly hard surfaces in the room. I want more bass as I use this 2 channel system for TV/Movies and music. The 714S's perform very well despite their small 5 inch drivers (treble can get "tizzy" on some source material) My original plan was to buy a NHT X-2 crossover to limit the bottom end of the 714S’s to 80Hz. This would free up power for more midrange/mid-bass/treble. I would then feed everything below 80hz to a Paradigm DSP or Ultracube sub (purchased locally). However someone local is selling lightly used Focal 816V towers at a VERY good price. Would the 816V’s 6.5” drivers and larger cabinets give me more of the bass I want without having to add a subwoofer? Will the NAD have enough juice to get them going? Both the 714S and 816V are rated at 91.5 db efficiency. I listen to all kinds of rock. Which plan do you fellow Audiogoner’s believe is better? The subwoofer/X-2 plan is slightly more expensive…but the 816V’s LOOK SO GOOD!
I think you'll still appreciate the impact that a sub will bring to your system especially with DVDs. I think the C325BEE is probably too small for your room. I think you run the risk of clipping with DVDs and damaging the speakers. I'd rather have the 807V instead of spending money on a larger box. Spend those dollars on a sub.
Just thought of another idea:
A B&K pre-amp with built in high/low pass crossovers, and a B&K amp in the 125 watt range. Keep the same speakers, and maybe add a sub later?