speaker impedance

They are about the same wiht any modern solid state amp.

I mean, in an ideal world, higher is better = low current and easier to drive, but from a practical perspective, impedance only matters a lot with tube amps or speakers that dip below 4 Ohms.

The only thing really important is their SQ. In a few cases you might find a better amp to drive them. 

Buy good speakers designed well and the designer will decide which impedance works best.

Are there any high impedance speakers out there designed with tube amps in mind? I see you can get 16 ohm compression drivers. A couple of 8 ohm woofers could be wired in series to give 16 ohm impedance. Doesn't seem to be a popular design choice.

If you're really serious about speakers that work with fleawatt amps, visit the Deja Vu website (the audio store in Washington DC, NOT any of the other ones!)