Dear Atmasphere,
Could you write a word or two about what you mean by "The trick with any tube amp is an effective 3-4 ohm capability...". Thanks.
Let's put it this way- tube amps in general do not perform as well on 4 ohms as they do on 8 or 16. Output transformers do not do as well on lower impedances due to increased turns ratios, which have inductive and capacitive effects that reduce bandwidth and absorb power.
OTOH, there are speakers out there that sound best with good tube amps and are 4 ohms. Maggies are an example. Not all tube amps with 4 ohm taps really work with the 4 ohm tap. For this reason, you are often better off with a set of ZEROs which are optimized for low impedances, while running the amplifier on a higher impedance tap.
In the old days Magnaplanar made some 8 ohm speakers which made the tube vs. transistor demo that much more profound. Going to 4 ohms has caused a lot of tube amps to cede some ground to solid state, but if the impedance issue is leveled, tubes easily win out. The ZERO is an easy access to this.