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

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.

I added a small Velodyne MiniVee to my Spica TC-60s and it created a balance that rounded out the sound nicely.  I pushed it with an Emotiva PreAmp and a couple of Yamaha amps bi-wired to the Spicas.  

Doesnt have ave the thundering boom that my SVS sub has on my theater system. Just good clean bass.  

Hmmm.>I suggest that you look for a 12-inch dual VOICE COIL system.Not seeing a lot of those advertised?Ben

To answer the question I seemed to have missed, after hearing a friend's system years ago utilizing one REL and liking it, I was looking around on Ebay for any reasonably priced REL and found a Q150e that was missing its grill for 200 bucks. Had the grill frame though, and otherwise was (and is ) perfect...stapled a new grill cloth on the thing and used it alone for a couple of years...great sub, 10" front firing, lots of adjustability. Next I found a mint Q108MKII (8" downfiring) also for 200 bucks, and its addition to the mix is great...note that a lot of the tedious yammering regarding proper setup or additional electronic crossovers, room treatment, etc., can be ignored by simply hooking the things up and moving 'em around some until they seem to work. I use mine in mono (hooked up together to the same amp binding posts) as they're simply not directional as bass position cues come from somewhat higher frequencies, and the subs in tandem work together to cure some room issues. Bottom line...use in a room with furniture, hook them up an enjoy them...I made my own cables with Canare Star Quad, but many used RELs come with a REL capable cable.

thanks wolf