It is all relative. However the relationship holds true; if you increase the source impedance relative to the load impedance the damping factor will reduce. In this relationship the source impedance includes the output impedance of the amplifier summed with impedance of the loudspeaker cable (mainly resistive).

You really need to plumb some figures in to begin to get some idea. Again the 5% figure is a reasonable rule of thumb. Loop DC resistance of loudspeaker cable (for a given length) less than 5% of minimum loudspeaker impedance.