I do have a tube amp (CAT JL2)that can handle low impedances, but it is expesnive and requires large transformers (the amp weighs 180 lbs!)and Atmasphere obviously uses a different approach. But nevertheless a higher impedance is always better for tubes and a smooth, linear impedance curve is important. I would think even a 4ohm nominal impedance would be ok, as long as the impedance curve was smooth, without large peaks and valleys at various frequencies - I think they call it impedance phase(?) or something. A discussion with your amp and speaker mfg always helps. I never understood much about the subject till Bobby at Merlin (speakers) explained some of this stuff to me and Atmaspheres contribution on the subject have always helped. The speaker in question seems to offer challenges to tube amps, even if they worked, they would "work" the tubes hard.