I've had three friends who recently added powered subwoofers to (1) inexpensive Mordaunt Short small speakers, (2) Dunlavy SC-1s and (3) Martin Logan Prodigy's. In each case there was a fair amount of work necessary to properly integrate the sub into the system, but the end result was very satisfying to each of them, the most noticable thing for them all being a much better reproduction of the space in a recording, even on material without a large amount of low-bass content. I was particularly surprised about the Martin Logans, which go quite low in their own right, being improved but they were, which leads me to believe that your JM Labs could be helped, even though they go pretty deep in the bass, particularly if you listen to large scale classical music. I would not advise doing it unless your room is able to handle low bass (I'd think more than twice if I had a small room, for example) and you're ready to spend the time and money necessary to integrate the unit, which is not easy. Your speakers are very good ones (as is the rest of your system!), and you would want to use a sub which would be able to match them in terms of speed and quality of bass reproduction or else your gains in bass extension could be outweighed by muddiness in the whole bass region (REL is probably a good choice--that's what my friend with the Logans uses). A sub with a phase control adjustment mechanism, preferably incremental as opposed to just in and out of phase, would also be useful in order to give you the best flexibility for room placement and lower to mid-bass integration. Finally, if you do get a sub, don't fall into the trap of making it too loud, as all of these three folks did initially; you should really not notice it when it's working and set up properly. Good luck!
