damping factor

Good answers above. There is an additional factor that often needs to be considered, though. Damping factor and amplifier output impedance are inversely related. Damping factor, as usually defined based on an assumed 8 ohm speaker load, equals output impedance divided into 8 ohms. To the extent that the speaker's impedance varies with frequency, that output impedance will affect tonal balance, by interacting with the speaker's impedance vs. frequency variation.

Many electrostatic speakers, for instance, have high impedances at low frequencies, which descend to low values at high frequencies. A solid state amp, having a high damping factor and negligibly small output impedance, will produce LESS bass and MORE treble into that kind of speaker, relative to a tube amp. The tube amp, having a relatively low damping factor/high output impedance, will produce MORE bass and LESS treble into that kind of speaker.

Conversely, many dynamic (cone-type) speakers have low impedances in the bass and mid-bass regions, and higher impedances in the upper mid-range and treble. Driving that kind of load, a solid state amp will provide a stronger bass response than a tube amp, and the tube amp may sound excessively bright.

As was stated, synergy with the particular speaker is key.

Regards,
-- Al

I usually don't like talking specs, just because of issues like this... Think about how much damping factor measurements can change depending on where you measure from. What if you measure at the output, what if you measure at the speaker? How can two amps have great bass control with one having a damping factor of 50 or 100 and the other at 400 or even 1000.... For me, even though this is a very legitamate questions, its useless, you've got to plug the thing in and take a listen. By the way, Slew rate is just as important(the amps ability to recover after a hard bass note). Good Listening, Tim

additionally, here is a good write-up on amplifier output impedance & damping factor:
http://sound.westhost.com/impedanc.htm

Here's a link you may find useful, that relates directly to the matter of damping factor and matching the amp and the speaker for optimal operation:

http://www.atma-sphere.com/papers/paradigm_paper2.html

There is no speaker made that requires more than 20:1 for a damping factor, and there are speakers that prefer a damping factor of 1:10 (usually high efficiency full-range speakers). If a speaker is over-damped it will produce less bass and what bass there is will be tight and 'punchy', without much in the way of definition. When the speaker is properly damped not only will you have impact but you also get definition.

If the speaker is under-damped, many people assume that the loss of control by the amplifier is responsible for muddiness, but this is not true. What is really happening is that the amplifier has a high output impedance relative to the speaker, and is making excess distortion, which is interpreted as a loss of control.

The size of the woofer cone has absolutely no bearing on how well the amplifier will be able to damp the speaker. The idea that the cone size makes a difference is a common myth.

One other myth is the idea that the amplifier has to be able to 'stop' the speaker as well as get it started. It turns out that this issue is minor- if you look at any audio waveform, you will see that they don't require this of the amp and speaker to any great degree.

The only area where this really comes into play is 'overshoot'- the idea that the cone will move too far on transient peaks. But most cones will see a fair amount of mechanical damping coming from their own suspension on transients, and when an amplifier is reproducing a transient its damping factor will be higher than the that same amplifier's damping factor will be at rest; this is really not much of a problem!