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

@casteeb 

I am a firm believer in subs regardless if its a two channel system or not. There is no sense in missing the information in the lower octaves simply because it's a two channel system. I guess Im just not that much of a purist. So if adding a sub gives you the bass response you want then sure as long as its done right I say yes. My amp does not have a sub output so the sub I use in that room is connected inline with the speakers, and I use the crossover on the sub to make it disappear. Sounds amazing. In my larger room I use two subs to balance out the room and it works well. I say get that bass. 

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If using an active crossover that also has capability to connect a sub(s) and vary crossover points for the main speakers and the sub(s), my understanding is to set the crossover on the sub to it's highest value and then use the crossover to set the actual sub crossover point.
Am I wrong about that?

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.