The Stereophile simulated loudspeaker load, which was mentioned earlier in this thread, has a minimum impedance of about 4.0 ohms at around 5 kHz. The circuit has been published, including details of the component values, and nominally represents a two-way closed box loudspeaker system (with a low-frequency resonance at about 70 Hz). Using those component values, it has been possible to compute this circuit's impedance as a function of frequency, and the results match the published measurements to an excellent degree. Some small differences are apparent, and these are probably due to component value differences between the theoretical circuit and the as-built circuit.
The plot below shows the computed frequency response functions of amplifiers with various values of damping factor (DF) when loaded by the Stereophile loudspeaker simulator circuit. The damping factors studied include values of 10, 15, 25, 50, and 100. The source impecance (Rg) is also indicated, and the impedance of the loudspeaker simulator circuit is also plotted. It is clearly evident that the variations in amplifier frequency response closely follow the variations in the impedance of the simulated loudspeaker load.
Once the damping factor is greater than 100, then the frequency response variations are going to be quite small, and can probably be neglected. However, at low damping factors, such as 10 or 15, which are in the realm of what tube amplifiers have, then the frequency response errors are much greater, and they are likely to be audible. One thing to keep in mind is that, when an amplifier has a low damping factor, it's frequency response will be affected differently by different loudspeakers.
For frequencies above about 200 Hz, it would be quite feasible to design a conjugate impedance network that could be used to equalise the impedance of the simulated loudspeaker circuit (or that of a real loudspeaker), and make its impedance much flatter. This combination would appear more like a constant resistive load to the amplifier. This would result in that portion of the loudspeaker sound pressure output becoming much less sensitive to the frequency response variations caused by amplifier damping factor. Conjugate impedance networks are already used in equalizing the impedance of individual loudspeaker drivers to remove their rise in impedance due to voice-coil inductance. These simple networks are usually known as Zobel networks, and they make crossover network design simpler in some circumstances.