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


The subwoofer can of worms.
I’d like to add some bass to my otherwise fine 2 speaker setup and have limited space so perhaps a single 10in?
After a few weeks of research the only conclusion I can come up with is - everyone agrees they have to be setup properly. Every brand and technology has their own following and most recommend using 2 (which I don’t want due to space concerns). My room is 12 x 13 ft with box raised ceiling.
After much research, these are the subwoofers I’m trying to pick from, for various reasons:
Rythmik FM8  - lots of hifi fans
JL audio e110 - fantastic support
REL T/9i - sort of a gold standard
KEF R400b - interesting technology, makes me grin. 

I've never heard any of these.
System:
Hegel H160
SF Concerto speakers on stands.
Ben
casteeb
With little space and you want very good performance, look at the James subs.  
The answer your question yes I think adding a sub really helps. 

I the JL audio E110 is a good choice. If you have a pre-pro or dac to amp you just sit the sub between the two and use the active internal crossover (not just a low pass but a true 24 dB cross-over). Works well and makes blending super easy. 

The only issue I see is two subs are way better than one. I find one sub harder to get a good blend but it can be done. 

I have two E112s and I would say it has been my best audio perchase. Don’t be scared to try a 60hz crossover point. Many will tell you it is too high but with a cross over it is not an issue. 
I have a 10" REL that I've used with several speakers over the years and it has worked well with both stand mount and floorstanders.  I have had the best results with low crossover points, like 60Hz or lower and  running the main speakers full range.   

My dad has Concertos with a Velodyne DD10 and it's awesome for a compact system.  

Some of the newer subs have built in EQ, I would suggest looking at those first.   I'm using a Velodyne SMS 1 to EQ my REL and that coupled with a Auralex isolation platform and it delivers awesome low end .  

JL and REL are great sounding subs in my experience
Considering the size of your room, the Rythmik FM8, JL e110 and Rel T9i are all waaaaaaay more powerful than you need. You’re literally going to have the volume on any of those subs at the lowest point possible or they will overpower your main speakers.

I’m running a Rel T7 in a room a bit larger than yours (~200 sq ft) and anything above the 6th click on the volume knob is overpowering.