Has anyone added a small subwoofer to 2 channel and been thrilled with the result?

If you've got an existing 2 channel small speaker setup the physics makes it impossible to reproduce deep bass frequencies below around 50 Hz with any kind of useful sound pressure level simply because those small drivers can't move the volume of air necessary so a subwoofer is a good bet.  But you run into the issue of crossover frequency and ensuring correct phase between the satellite speakers and the subwoofer in the crossover frequency range.  Try to get a subwoofer with a built in crossover i.e. one that you connect the 2 channel small stereo speakers into and then the subwoofer to the amplifier.  This will ensure correct phase and crossover.  Adjust the subwoofer level for a well balanced sound, maybe just a little extra.  You may be pleasantly surprised how much bass content you've been missing in your music.

No.  But only in my trick I am using a box with boss 10 inch cheap driver and is not too bad.  It is connected to 1 600 watt Alpine amp.  The model before last.  My theater is the same size as your room and running 2 2000 watt dual 18" SRX cabinet and I am quite pleased with the performance.  For the truck I havent got around to installing the second 10" sub and amp configuration.  I just need a couple hours to wire it in.

+1 on REL. The best decision I made was to add a REL T5i sub to my system (room size 12x16 feet). I was on the verge of upgrading my Dynaudio Contour 20 bookies to the bigger Contour 30 floorstanders due to a slight lack of bass (noticeable with jazz and electronica). I realized upon auditioning the Contour 30’s that I preferred the imaging of the smaller Contour 20’s and adding a sub would be a more affordable option as well. Turned out to be the best decision as the C30’s would have been overkill due to my room size. Once I got the REL dialed in it brought my system to another level. Hope this helps! 

i use these musical subs with great success.
http://www.tbisound.com/dsp_products_subwoofers.asp

@lance-baker The op stated he wants to 'add' bass to his 2 channel system, not win a SPL competition. DVC setups are unnecessary for what the op is looking for. The op also stated his room is 12'x13' so it's not going to take much to add some additional 'weight' to the sound in that size room.