05-27-11: Elizabeththere she goes again..............giving explanation on technical subjects she has no/little understanding!! Very similar situation to the "what is tube bias?" question posed not long ago by some other member.
The damping factor is the 'control' the amp has of the woofer movement.
When the amp has a signal that moves the woofer and then the other way, the damping factor is the control the amp has to "HOLD" on to the cone and not allow it to continue going in the first direction, instead of obeying the amp and moving in the opposite immediately that the new signal wants. (sloppy explanation but it will do...
So "IF" your speakers have a BIG, 12" or maybe even a 10" sloppy woofer, the higher damping factor amp will 'control' it better, and that big sloppy woofer will behave better, giving a more solid punch, with less overhang and less woolly sound.
Where if you own a set of speakers with a single or pair of 'tight' 8" or 6" woofers, then the damping factor will not matter nearly as much, and other factors will overshadow the damping factor.
Generally it is said ANY solid state amp has enough damping factor to control any modern speaker system woofers. Only if you had some old speakers with giant woofers..
And the place where damping factor may be a little too small for some speakers is in a tube amp. Although the combo of a tube amp and big efficient speakers is usually heavenly, even if the bass is a little sloppy.
So all in all, you do not have to worry in any way about that particular spec called Damping factor. It is used to sell stuff, particularly amps which 'just by chance' have a particularly BIG damping factor, but it is a nearly meaningless spec.
Size of woofer cone has no bearing amp's ability to control woofer.