Subwoofer Suggestion

I hate to throw water on the party but if all you have to spend is $1,500 you are better of not doing a subwoofer. For that money you will never do your Focals justice. The overall effect will be worse than no subwoofer at all. A subwoofer system to match those speakers would be at least $4000.00. I would hold off till you have more to spend.

Nonsense. By what metric does $ = ♥?

A decent sub, properly implemented, increases realism beyond all proportion to the frequency response. 

Dual subs are better than one. Subs output into the mid-bass. A single sub collapses the bottom so the soundstage is like a truncated and inverted pyramid.

Most sub installations are abysmal. It takes skill and work to integrate properly. As a pal once opined "EVERY other  sub I've heard just boomed." 

First of all you need a continuous phase control, polarity control and multiple crossover slopes.

Second you need to roll the lows out of the mains. Doubly important with ported mains. This can be a simple 1st order capacitor crossover to the main amp.

Third, a sub with a control app to equalize the bottom to the room can do wonders.

Among others, the Martin Logan x series have all of the above. 

see  http://www.ielogical.com/Audio/SubTerrBlues.php for setup and general sub info. The Finally graph shows the proof of the pudding is in the eating.