what is dynamic headroom?

Aleph 5 50 nominal is the damping factor
Aleph 3 100 nominal is the damping factor
SF Power 2 100 damping factor
ML 33 greater than 800 @ 20 hz, this is a very good figure to look at as this is typically where you want the greatest control of the drivers, in the low bass region. The Aleph products because they don't over control drivers are considered more tube like. Tubes also don't control drivers as well as SS.
This didn't answer your question about DH but should give a reference on Damping Factor.

wow! thanks for all the great responses. i love this site! the B&K's dampening factor is 100 at 50Hz. that's all the info that the specs give, but it looks like i do not need to worry with the specs and see how she sounds.....
thanks,
fISHCALL

Gs: "As far as damping factor is concerned, a ratio of speaker impedance to total output impedance greater than 10 will adequately control most cone drivers."

Does this mean that an amp with an output impedance of .1 ohm will be able to control a speaker with an impedance of 1 ohm ? That is a 10:1 ratio. : )

Other than that, the higher the output impedance of the amp and / or the lower the impedance of the speaker, the more the damping factor is reduced. The lower the ratio of damping factor, the higher the potential for the speaker to load down the output of the amp. This can result in higher distortion levels, frequency response abberations, poorer transient response, etc... Given typical SS amplifier designs, an 8 ohm load would be more desirable than a 4 ohm load in terms of "control" even though power transfer is reduced. As you go lower in impedance on the speaker, the amp is not only working harder to pass more current and dissipate more heat, it could become less linear depending on the design. When you start combining low impedance with low sensitivity AND high levels of reactance in a speaker, you really hurt yourself in terms of limiting the amount of amplifiers that you have available to choose from. Unfortunately, some very good speakers combine all of those attributes, making the need for "tank-like" amps a necessity for use with those speakers. Sean
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Hi
Here are three decent online articles on damping.
Probably in the order they should be read if you are not familiar with the topic.

Murphy: http://www.trueaudio.com/post_013.htm
Pierce: http://www.diyspeakers.net/Articles/Richard%20Pierce%20DAMPING%20FACTOR.pdf
Tomcik: http://www.otlamp.com/articles/tomcik/index.html

Clueless: The first two articles are interesting to me because they seem to imply that differences in damping factor shouldn't matter much in audible terms, which could be seen as somewhat contradicting the notion of critical damping as explained in the third article. Of course my own listening experience with various amps possessing different damping factors can't confirm or deny any conclusion decisively, since many other factors are always in play (such as the impact of negative feedback on amplifier sound) without being held constant. But thinking about this stuff always puts me in mind of resistive loading of MC cartridges, which I see as an analogous situation, and in that case I *have* heard audible differences corresponding to system resistance changes with everything else held constant, and those results would seem to support the notion of there being underdamped, critically damped, and overdamped conditions possible.