I can't speak to the servo sub question, but I think the primary difference between those two models is the tweeter. I have heard both, but never in the same room at the same time.
Some of the X series that started as GR Research designed products for the now defunct AV123 and were given a second wind via Danny. I may be biased, but for the tweeter in X-LS is the same as X-Otica, which speaks to it's abilities pretty well, methinks.