Recently I read somewhere that tube amps are not good at driving speakers which have a wide impedance curve, say if the impedance swings from 3-20 Ohms across the audio range.Not many speakers have such wild impedance curves. The SoundLab speakers (which I own) do have such a rise in the lower freqencies. This ultimately puts less stress on the amp in the area that often causes the amp the most challenge.
Well designed tube amps do a better job to provide their rated power across a wider load impedance range. I am sure Atmasphere will chime in here to cover the technical details.