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
I've got a JL e110 matched with my JA Pulsars. Sonic Nirvana in my small room of 10x11x8. I like the fact that the e110 lets me use the internal crossover ... really lends to holographic imaging from the speakers since they don't have to carry the bass load.
Yes....and depending on the recording, sometimes don't need it.  Really added a subtle improvement to some of my Zappa....
inventor of THX
THX is a specification to ensure adequate playback capability for Lucas films. See  https://en.wikipedia.org/wiki/THX

Linkwitz-Riley is not the be-all and end all of XOver options. It has it's uses, but it is by no means universal. In some applications asymmetric slopes and phase inversion are preferred, while still others symmetric 2nd order may be best. Room and components play a large part in the choosing
As with many terms, "subwoofer" no longer means sub. It simply means "center woofer" or "dual-channel woofer". The original meaning from the "High Fidelity" era was "ultra-low frequency woofer", such as a 15-inch in a 6 cubic foot enclosure. Your main speakers now don’t actually have any woofer. The have what is advertised as 5.5 inch "woofers". If woofer comes from woof (sound made by a very large dog), those are more like "yelpers". It is not possible for such small drivers to move enough air at low-frequencies to be significant. The Rythmik FM8 has "dual" 8-inch woofers, so only a tiny step above what you now have. The KEF R400b has a pair of 9-inch drivers, hardly an improvement. The JL audio e110 is a 10-inch, still not serious bass power. All of these depend on built-in high-power amps to push the driver to high dB levels in the 50-150 Hz range, and that means louder bass that you already get smoothly from your current speakers. I suggest that you look for a 12-inch dual VOICE COIL system (one driver with two voice coils) in either a 2.5 cubic foot or larger cabinet. That type can provide smoother response and less distortion in the 20-100 Hz range, supplementing your present system well.
I added a small Velodyne MiniVee to my Spica TC-60s and it created a balance that rounded out the sound nicely.  I pushed it with an Emotiva PreAmp and a couple of Yamaha amps bi-wired to the Spicas.  

Doesnt have ave the thundering boom that my SVS sub has on my theater system. Just good clean bass.