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
Has anyone tested the Dynaudio Sub 3 or 6?For just extending the lower frequencies on a 2 channel system they look viable.
Just to share my experience:  in my large L-shaped room, my GoldenEar Triton 7's were just not providing the depth/weight of sound I wanted.  After doing a lot of research on upgrading the T7's and not coming to any conclusions, a pair of discounted subs caught my eye (SVS SB12-NSD subs, substantially discounted at $400 each, as other have mentioned here).  Knowing that SVS allows free returns, I purchased a pair, just to see if subs would help give the bigger sound I was looking for.  For me, with one placed next to each Triton 7, they definitely do, and I now consider the $800 I spent on them to be pretty much as worthwhile as the ~$5K I've spent on the rest of my setup (T7's, GoldenEar center channel, Marantz preamp, Emotiva power amp, single 15" Hsu sub).

I don't have the knowledge/experience to offer any suggestions on sub comparisons or placement, but to me, these two entry-level subs made a very large difference that my single 15" Hsu, perhaps because of positioning, had not.
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