What you`re seeking is`nt a characteristic of tubes.This quality is offered by some solid state designs. This does`nt ensure better sound necessarily in all circumstances.Really depends on the speaker being driven.
I agree and emphasize that tube power actually seems to increase against higher impedance loads not the other way around. I understand that this is a lack of higher current being made on demand by tubes in that face of low impedance. If you want more watts output then you need more tubes and overall bigger tube amps to get that. This is one of tube's limitations, which in my mind are made up for by the imagery and tone tubes are capable of. I have not heard with SS amps that equal it. Some fine SS amps do image pretty well but not to the extent a tube power amp does.
another simple way to explain this may be: Tube amplifiers may have a typical plate voltage in the order of 450 volts. This voltage is not directly suitable to drive loudspeakers because the output impedance is very high. Not to mention shock risk to the people setting up/using the gear. An output transformer matches the impedance of the tube circuit to the value needed for the speaker. 4, 8, 16 ohms and 25v and 70v lines are usually what one encounters. The 25 & 70 volt lines are for commercial systems...stores, outdoor arenas where 4, 8, or 16 ohm systems would loose very significant output power over long distances. These transformers maintain the efficiency or equal output power at all impedance values.
McIntosh has used "autotransformers" for decades in their solid state amplifiers isolate their output stages and therefore produce equal output at all typical impedance values.
Solid state amplifiers without autotransformers will behave in such a manner that output power varies directly with speaker load, that is more power at 4 ohms than 8, and less power at 16 ohms than 8.
There are/were "line matching transformers" which enable solid state amplifiers to operate in 25 & 70 volt systems