Nice speakers NB. I have a pair of XLS Encores that Ruben made for me a couple of years back before he disappeared. I'm currently pairing them with an X-CS center channel as the front array of my HT. I've just received my second Rhythmik F8 subwoofer that also doubles as a stand for the XLS Encore. Even driven with a 50 wpc Marantz AVR they really get up and go in my room. I use them crossed over at 80hz, so that may help (along with 1,200 watts into 4 8" subwoofer drivers) them give me all the dynamics I need.
I have a local shop that does nice mid fi stuff, including KEF and Marantz. I've had a chance to hear the KEF LS50's driven by a similar Marantz integrated amp and can say that I preferred the XLS. They aren't as flashy as the KEF but definitely are less fatiguing.
I haven't tried them with tubes yet but have a 12wpc Inspire amp that might make them happy.