So true. Rudimentary physics.
Also, one Watt from a valve amp is the same power as one Watt from a class D amp, or even one Watt from a kettle. Again, (very) rudimentary physics as a Watt is a standard measure of power.
However, what I don't know is if one Watt from a Bass 400+ valve amp generates the same sound pressure (dB) as one Watt from a class D amp. I guess that depends on the speaker system being used, which convert electrical power (Watts) into sound pressure (dB).