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

Like danatek2 said....
I was searching for a relevant sub that would compliment a pair of Maggie 3.7i
ribbon speakers and tried a half dozen of the more popular (and expensive) offerings.  All sounded muddy and out of sync as the Maggie's are very fast and accurate.  At an old time music lovers suggestion, I was told to look for a short throw,  sealed sub no larger than 10" to make sure it would damp quickly.  Found a Martin Logan Dynamo 700w and that was it!  Within minutes of trying it out in my home system I knew I had found what I was looking for in bass re-enforcement.  Fast, accurate, with no boom or after tones ruining the main body of the music.  Best part?......it was $600 bucks.  Subs have to have synergy with your room and existing equipment.

Having gone through 3 or 4 subs that filled out the bass but whose integration with the main speakers always seemed so-so, I finally found the seamless integration I was looking for (with SF floorstanding speakers) through a software-configurable Elac Adante SUB 3070.  I think two factors were responsible for the difference: the smartphone microphone-assisted setup that is then further customizable via the Elac app and the design of this sub that incorporates 2 diagonally-opposed 12" woofers.  This sub is designed for music, not AV, and aided by drivers that point into the room diagonally, it never hints at boominess, and it reveals little of the sub's positioning.  This is not a small sub, but it is certainly smaller than two 10" subs, and given the lows it can reproduce, making room for 12" drivers is certainly worth it.  The fact that its cabinet is furniture-grade also eases the inconvenience of accommodating a big box. 

I added a Gallo 10" sub to my German FJ  2 Way loudspeakers. It is 300 watts and goes down to 18Hz. It is fully adjustable and I was able to integrate it perfectly with the speakers. I set its crossover setting to 50Hz I connected it with the 2nd set of outputs from my preamp. I didn't have a lot of room for a sub, so it being only 10.5" wide it fit nicely.

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!