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

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

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.