Would you choose 1 ou 2 subwoofers for a stereo set?

In a room that size you need at least two 12" subs. Highly recommend the Martin Logan Balanced Force 210. There are two 10" drivers in each. Get two of them. They are a much better value than JL subs which are overpriced for what you get. I suggest you use a two way digital crossover like the MiniDSP. Your amp has all the ins and outs to run it. If you get a MiniDSP SHD you get the crossover and Dirac Live room control which will blow your mind!

@tvrgeek 

That is lay intuition tv. The speed a woofer can move is indicated by the upper limit of its frequency response. Most 15" subwoofers make it easily to 500 Hz. The transient response of a sub driver has to do with the relationship of its motor power to the mass of it's moving system. One issue is certain, the distance a driver has to move is directly proportional to the amount of distortion the driver produces. The smaller driver has to move farther to produce the same volume producing more distortion. The main disadvantage of larger drivers is the size of their enclosures. I use two 12" drivers per enclosure which is like using one 15" driver. I did this for packaging reasons. I do think once you get beyond 18" the cone becomes more difficult to control and pistonic motion can be lost. 

@markalarsen 

The new DEQX units which will be along shortly are insanely more user friendly and considerably more expensive. They will not require an expert to set up. Their build quality has gone up significantly. The Pre 8 has a 4 way crossover with 8 DAC channels! It will retail for $10,995.00. 

@ditusa 

I watched a 21" driver under a strobe doing a snake dance at relatively small excursions. A good 18" driver should be OK. The problem is the size of the enclosure needed to house the thing. I would like to be able to see my screen!