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

Dont know the Kef or Rythmic .. know JL is a respected sub company.
Had a Paradigm 2200, nice. But what a step up for me going to Rel. My vote is for REL!

I added a 328 to my Monitor Audio small Gold 10 monitors, then upgraded speakersto tower Aerial 7b's. In both cases the REL has been a nice cleaning up and defining to the bottom octaves.

I liked the philosophy of signal transference from amp to sub regarding signal timing as it relates to speaker woofer & sub unity versus that from the pre-amp to sub. The idea floated is can we assume that pre-amp to sub via LFE is timing coherent with whats being sent thru via pre-amp/amp/speaker to eliminate a potential "time smear" (my terminology). It also made sense to me that the amps output would more likely align the sub to subtle sonic characteristics found in the amp.

Just my experience between the LFE input use w/the Paradigm versus the amp output to the REL gave me a sense there maybe something to the concept.

Take a look at the SVS sb3000 - its their newest model that allows dsp control from an app on your phone. You can download the app to check the settings out before purchasing anything. For 1k, seems like a great product - its next on my list to check out!

Monolith 10in is a nice small sub.  

I bought a Rythmik f12, i have been running them along with my Scientific fidelity Crown Jewels. I have also ran them with my Acoustat Monitor 3's crossover set around 60 and it was surprisingly coherent and musical.

Upgrade from a Def tech sound field 15 to JL F112 v2 and realized everything I had been missing. I could not be happier