Atmasphere's description of the various models is dead-on right. I was an original owner of the first version of CLS stats, and followed the so-called upgrade path step by step to the CLS-Z. The original CLS was a very easy load for tube amps. I used them mostly with an RM-9 Music Reference, which was a great match, but I also tried them with some Dyna mods (Mk. 4 and ST-70) and they worked great as well. The CLS-II was quite a different story. It had a punishingly low impedance at some frequency due to a notch filter in the crossover. Supposedly it made the speakers more compatible with Krells and Mark Levinson amps, but it didn't like tubes at all. Things got better with the CLS-Z. Unfortunately for me, I was fed up with the CLS by that time. I found I was no longer playing music I liked, just because it didn't sound so good on the CLS. I opted instead for cone speakers that are more universal in terms of music playability, but it is true that the CLS has a special magic on the right types of recordings that none of the cone speakers I've used since then can equal.
Dave
Dave