While it is true that 'speaker sensitivity plays a role in required amplifier power, it is also true that amplifier power capability plays a role in distortion. An amplifier will exhibit lower distortion at low output levels than at high output levels. Distortion will often rapidly increase as the amp's rated power is approached. Linearity is usually much better at small signal levels, hence lower distortion. This means that at a given output power level, well-designed high-power amplifiers typically exhibit lower distortion than lower-powered ones.
I run two channels of four Harmon Kardon Citation II tube amps each that, on the bench, deliver over 500 watts/channel with 0.05% THD at 1 kHz if line voltage is held at 120VAC. This means at normal listening levels, each amp is operating at or close to small-signal operation. Since the 'speakers driven by these amps are B&W 802Ds, I am glad I have the high-power capability.
Normally, solid state amps will deliver twice the power to 4-ohm than to 8-ohm loads. However, power supply limitations can limit 4-ohm output. Tube amps generally deliver the same power at 4- or 8-ohms provided the secondary winding of the output transformer has a tap to match the 'speaker's impedance.
By the way, at age 77, it is getting to be a big burden to carry a 60-pound tube amp from the first-floor listening room to the second floor test bench every time I have to change a tube. I do so because I set the output tube balance for lowest THD. If anyone wants to make an offer for 10 Citation IIs (I have four spares but want to keep a pair for other use), please let me know.
Norm