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
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.
I’ll just say Hegel is a Norwegian Company well known in the European Audio Community and just getting a solid foot hold in North America, especially here in Canada. 
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.