Wikipedia has a pretty good explanation here so I'm not going to rewrite it (at least not completely ).
I'll try to keep this not too technical, but to really understand what's going on you need to look at the diagram on the WP page. The amplifier is a voltage source, which is followed by the amplifier's output impedance (modeled as a resistor Zs). The speaker cone has mechanical mass and therefore it tends to overshoot the required position and the suspension wants to keep it oscillating around the final steady state position, if the ampifier output voltage first moves and then stays level.
These oscillations make the speaker work as a generator, which creates an oscillating voltage, which the amplifier then "sees" and tries to compensate (as it is a voltage source, it tries to keep the voltage at the left side of Zs related to the control signal = audio as closely as possible). But, the speaker sees the voltage at the right side of Zs. Looking from the viewpoint of the speaker, this creates an unbalanced state, which reaches equilibrium the faster the smaller Zs is = better speaker control.
Coming to your examples, I believe the 3000-2 is the only one to have AC coupled outputs (routed through large series decoupling capacitors). Therefore, its source impedance is higher and damping factor thus lower. But as you say, many consider it better than the 2400 - which just goes to show that damping factor isn't a very important criterion with modern amplifiers.