Let's stay out of the lab. Last time we went there it got messy
As always it's best to refine the language as to who provides and who needs. Amplifiers provide both voltage and current. Speakers require or use this voltage and current.
A mostly resistive speaker like a Magnepan looks like a resistor (long piece of aluminum wire) with no significant inductance, capacitance or back EMF. It's about the easiest thing to drive and closest to the load resistors we use to specify and measure amplifier power.
Tube amplifiers have output transformers that provide different "impedance taps" where the voltage and current are in the optimum relationship for the indicated impedance. The RM-200 can provide the same 100 watts into 8, 4, 2, 1 ohm because it has those taps. The sound of the amp is virtually identical on these taps when loaded with the marked impedance. It is permissible and often desirable to put a 8 ohm speaker on a 4 ohm tap. I call this "light loading" and first defined it in the RM-10 manual. In this configuration the amplifier provides 1.4 times more current and 1.4 times less voltage. As long as this voltage will play the speaker to the desired level without clipping this configuration favors the tube life, lowers distortion and noise, provides 2 times more damping.
Transistor amps with output transformers (Mc Intosh) behave as above and it's the transformer that makes this happen. Mc Intosh wisely drives the transformer, autoformer in this case, from a very low tap around 2 ohms. Thus the transformer steps up the voltage by 2 times to the 8 ohm tap and 1.4 times to the 4 ohm tap. Transistors love high current much more than high voltage. Generally output transistor power capability falls rapidly above 80 volts due to second breakdown. One hundred watts into 8 ohms is roughly where second breakdown starts to reduce transistor capability. Above that power the designer has to use lots more transistors, or MOSFETs, or protection circuits that become quite problematic when driving a reactive (non resistive) load like an electrostatic speaker.
The better speaker makers and Stereophile speaker tests give impedance curves and phase angles of those impedances. Phase angle in this case has nothing to do with the often misused term "phase shift". In this case the "phase angle of the impedance" serves to tell us the capacitive and inductive components influence the impedance curve. A Magnepan has a zero phase angle. An electrostatic speaker that dips to 1 ohm has a 90 degree phase angle. This means that the amplifier will be providing the most current as the output voltage goes through zero. As far as the output devices (tube or transistor) are concerned this looks like a short. Tubes will run out of current, both will run hotter and load line limiters that protect transistors will often cut in sounding very bad. This "current clipping" sounds very much like voltage clipping.
