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

In a smaller room like that I would be looking at REL T5 or Sumiko S5. The larger subs will be difficult to tame without DSP or some type of setup with 2-4 subs to flatten out the room response.

I also don’t think that the difference between high level and line level is enough to be concerned about. I have had good results using both with REL and Sumiko. 

With KEF Ref 1s I'm using a pair of Velodyne HGS-10s low-passed at 40 Hz with 24 dB/octave slope by a SMS-1 bass manager that also does acoustic room correction.  The result is an excellent full-range sound.

Casteeb, I use the JL e110 with 2 way stand mounted speakers and it sounds great. My room is 13 x 18 x7 and the 1 sub is more than enough to fill the room. I run the speakers at full range and cross over the sub at 45Hz. My speakers are rated to around 38Hz.  The sub can be placed on either side of one speaker but at least 1 foot from the side wall. My sub is on the outside right next to my left speaker on the same plane around 45" from the front wall to avoid any booming.

The key is to set the phase on the sub which is quite simple. Reverse the cables of your speakers so they are out of phase with the sub. The play music with drums and put your ears between the speaker and the sub. Start to change the phase on the sub until the bass is basically cancelled or very low. Reverse the cables back to normal. Go back to your listening position to determine the amount of bass coming from the sub. You don't want the sub to overwhelm the speakers but to blend in. This make take a few adjustments but the entire process should take less than 15 minutes. Once you think its ok, then turn the sub off and compare it with the sub on. As I stated you don't want the sub to overwhelm the overall sound but fill in the music. The volume on my sub is only set to the 9 o'clock position but its enough to fill the room but feel the lower notes. Best wishes.

I really appreciate all the insight from folks who have implemented sub based systems and have empirical results to judge by.Very helpful for a newbee.
Thanks guys.

With the SF Concerto, rolling the low end from the mains and adding a sub will increase the 'low end precision'. Ported speakers have poor bass transient response. Consequently avoid ported subs if you want good bass transient response. Hence, a crossover for the main amp is required. It can be a simple capacitor.

IMO, a sub must have a phase control, 0 to 180° and a phase inversion switch. Crossover contour with multiple filters is a plus.

With proper phase integration, the increase in realism is way beyond the extra octave.

See http://ielogical.com/Audio/#Basement and http://ielogical.com/Audio/SubTerrBlues.php for some sub integration experiences and tips.