If for example a 10W 300B single ended amplifier is played it may sound great and full at 6W.
But if it is used at night, the low output used to not disturb others may be much less than 1/2W.
It is a pity as it is far below the point the amp gets its juice out.
If this is correct then a constant impedance 8Ω resistive attenuator could be connected between amp output and speaker input.
There will not be any change in power consumption. A given class A (electrons flowing all the time) amplifier has the same power consumption no matter how much driven it is. In fact the more it is near its maximum power output the more it cools down. More of the constant power supply is given the the load and less is dessipated as anode electron collision heat in the device. The anode heats up because electrons collide to it and loose their energy to it. Much of this energy is converted to heat by increasing the thermal motion of the anode atoms and other radiation. Perhaps this is why one can see this blue out of this world color outside of the 300B electron tube. Are also soft X rays emitted?
Another way to make the tube sound full and loud at low output but reducing power consumption may be decreasing the anode voltage. So the system is still at its limits but at a low power output. The electron tube should still sound loud as it is producing 2nd harmonics etc, and is instateniously peak limiting as a perfect mastering amplifier with no artifacts.
References:
Applied Electronics - T.S. Gray - M.I.T.
Tubes vs Transistors, is there an audible difference? - Russel O. Hamm - Journal of the Audio Engineering Society
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