Peter,
Not to start another passive vs active like debate. The XS series can be described this way...any superlative you can come up with for the XA series, the XS series does it better and by a significant margin. The amps are quieter, have better bass grip, are more defined and extended throughout the frequency range. They are more transparent, sound stage better and just sound more musical. At this level, XA to XS, The improvements are more incremental. Whether that increment is worth more than 3 times the price of the XA 160 for example is for you to decide. It was more than worth it to me, especially since I am fortunate to have the resources to buy bleeding edge components. At this stage with my system, it does not make sense to investigate any products that are not the reference product for the given manufacturer.
I agree with Christian's assessment. One additional comment I would make is that layering in the soundstage and the XS300's ability to portray instruments and voices as a 3 dimensional entity continues to amaze even after having owned them for more than a year.