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

@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!

As others have said, absolutely go with two.  Your room isn’t all that big, and two E110s will be more than sufficient as they go down to 23Hz -3dB.  If it was me I’d get two Rhythmic F12Gs as you can get two for just a little more than one E110, and the F12G goes down to 14Hz.  Either way, two good subs are better than one bigger one especially for music.  Just my $0.02 FWIW. 

@gladmo: NoRez is a great product. Since I built sealed enclosures for the Rythmik 15" kit, I braced the Hell out of them: 1.5" x 1.5" Baltic Birch ply braces (doubled pieces of 0.75" x 1.5") every 6" inches, in all three planes: front-to-back, left-to-right, top-to-bottom. The braces take up a lot of internal volume, so the boxes ended up being rather large (for 15" subs): 24" tall and deep, 18" wide. When you do that NoRez is not necessary.

Danny Richie recommends NoRez for even his GR Research open baffle subs and loudspeakers, on the large unsupported (structurally) panels.

I use two but I have used a distributed bass array of four placed asymmetrically within the room which eliminated all the rooms standing wave bass modes and provided immediate sense of timing or room loading.

I've since manage a similar but not as total effect with two bass mode carefully positioned subs.

JL Audio makes a potent high quality product. Personally I found their F113 automatic / frequency only optimization lacking the flexibility of added  manual adjustability. This Youtube shows the Quality.

https://www.youtube.com/watch?v=GQ10cQ9O2L8

FYI, since REL are -6dB "Sub-Bass Systems" their frequency rolloff begins so early they usually don't excite a rooms standing wave bass modes allowing for greater flexibility of positioning at the cost of actual subwoofer extra low frequency presentation. Claims of superior musicality are purely subjective.  

definitely a pair