Banjofan, you might check with the guys over at the Magnepan Users Group on AA, otherwise known as the planar speaker asylum. There are a lot of Zero users over there. A number of guys there like to run low, and or, difficult impedance loads, with tube amps. Planars and tubes are where my heart is in many ways. I just happen to like the speakers I have now and they just happen to be monkey coffins(that's MUG lingo). If you just happen to love the Cremonas, and I can see why you wouldn't. They're a work of art, and so is the V12. These might be something to find out more about. Here is the link to MUG. Do a search for "autoformer", I don't understand why but "Zero" will bring up nothing, yet the results for autoformer is peppered with the word "zero", go figure. You can regester there and ask a general question about what current users think of them.
I haven't heard any negative reports about the sound that I recall. Everything has it's own contribution to the sound. I can't see that they can be more colored than many of the speaker cables people seem to like. People with some damn exotic stuff seem to love these things. If they add any coloration it may be out weighed by what they give you in return.