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

The integration of a sub(s) is one of the most popular topics discussed in these forums. Beside the choice of a particular sub the most important is placement in the listening environment. Each room is different.
You didn't state your main objective to add bass to your sound? Some like a powerful/floor shaking bass and others just want to fill in missing sound from the capability of the main speakers. What's your goal?
Maybe I missed something in your original post but I don't know what amp and speakers you are currently using? Whether floor standers or monitors. And, what type of amp you're using to drive them?
I also have a 2 channel setup using monitor speakers. I didn't want overpowering bass but just enough to fill in the complete sound. A compromise to using full range floor standers. With my setup I use the speakers with 2 10" subs. My integrated amp doesn't have any type of bass management capability. I use an Outlaw ICBM bass management unit. You could call it an active crossover. What it does is to send frequencies that the main speakers can't handle to the subs...thus relieving the mains from duty they can't handle. That's important.
Sub placement is the most important factor. If you just want to "round out" your sound then one or two subs will do the trick. Just be careful of crossover frequencies between the mains and sub.

>I’d like to add some bass to my otherwise fine 2 speaker setup
>System:>Hegel H160
>SF Concerto speakers on stands.
Ben

With little space and you want very good performance, look at the James subs.  

The answer your question yes I think adding a sub really helps. 

I the JL audio E110 is a good choice. If you have a pre-pro or dac to amp you just sit the sub between the two and use the active internal crossover (not just a low pass but a true 24 dB cross-over). Works well and makes blending super easy. 

The only issue I see is two subs are way better than one. I find one sub harder to get a good blend but it can be done. 

I have two E112s and I would say it has been my best audio perchase. Don’t be scared to try a 60hz crossover point. Many will tell you it is too high but with a cross over it is not an issue. 

I have a 10" REL that I've used with several speakers over the years and it has worked well with both stand mount and floorstanders.  I have had the best results with low crossover points, like 60Hz or lower and  running the main speakers full range.   

My dad has Concertos with a Velodyne DD10 and it's awesome for a compact system.  

Some of the newer subs have built in EQ, I would suggest looking at those first.   I'm using a Velodyne SMS 1 to EQ my REL and that coupled with a Auralex isolation platform and it delivers awesome low end .  

JL and REL are great sounding subs in my experience