If you are planning to use a tube amp, do yourself a favor and avoid low impedance speakers. Your amplifier dollar investment will thank you.
Tube amps using output transformers can lose as much as an octave of bass response going from an 8 Ohm load (using the 8 Ohm tap) to a 4 Ohm load (using the 4 Ohm tap) and the output transformer will run hotter (since more of your output section power will be heating it, not a good use of that power). The amp will make more distortion too and the speaker cables will play a far more important role (and will need to be heavier).
In the old days when tubes were the only game in town, speakers were much easier to drive- they were more efficient. Efficiency is helpful because you get less thermal compression in the voice coils- a more vivacious sound results (IOW this is both measurable and audible).
So a speaker of 8 Ohms (particularly in the bass, so be careful of speakers using dual woofers that might be rated '8 Ohm compatible') or better yet 16 Ohms, along with higher efficiency (+90dB) will help you get the most out of your tube amplifier investment (this is helpful for transistor amps too, since they also make less distortion into higher impedances...).
If you are planning an SET, its helpful to be aware of what you're getting into: If you want the most out of an SET, your speakers must be so efficient that the amp never makes more than about 20-25% of full power. At power levels above that, the higher ordered harmonics start to show up on the transients (where the power is), causing the ear to perceive the transients as 'louder'. This is why you read so much about how 'dynamic' SETs are; it really distortion masquerading as 'dynamics'. To avoid this you need a speaker efficient enough to prevent this phenomena occurring. Of course, SETs have troubles with bandwidth when built to make more power; usually the bass is curtailed in favor of higher frequencies in the output transformer design. This is why the lower powered SETs are known for better sound.
My advice is that the mark of a good system is that it doesn't sound 'loud' even when it is. The dynamics comes from the recording, not the electronics! To that end, the speakers must be easy to drive so that the amplifier is spending most of its time loafing.