what is dynamic headroom?

The dynamic headroom bit is very well explained in the link provided by Zoya so I will not repeat it.

Re. Damping factor: Damping factor is defined as the ratio of loud speaker impedance to amplifier output impedance. Nominally & simplistically, 8 Ohms is used for the speaker. So, if your B&K has df=100, it means that the B&K output impedance = 8/100 = 0.08 = 80 milli Ohms. This is pretty good.
B-U-T.......d.f. varies with frequency!!! So, in your B&K specs does it say d.f.=100 @ what freq??? Usually lesser amps have poorer d.f. as the freq. rises 'cuz they were too cheap to design a robust power supply!! This of course keeps cost of the amp down. Really hi-end amps have high d.f. across the audio band but then they are very heavy (power supply) & they cost much, much more.
What d.f. really does is that it controls or DAMPS the woofer driver in the speaker. By the amp presenting a very low impedance to the woofer driver, the amp provides a near short-circuit path to ground for the woofer back EMF. This allows the woofer driver to stop its pistonic motion after the bass note(s) go away in the music signal. We perceive this as tight, articulate & fast bass. When the d.f. is low (like in many tube amps) we feel that the bass is wooly, slow & boomy.
So, 100 would be a very good # if it is across the audio band ( tho I doubt it for B&K!).
FWIW. IMHO.

Zaikesman,

Good info in your post borne out of experience I note & point well taken. Thanks for sharing the same.

BTW, I wrote some basics on D.F. without meaning to write how much is high enough (as Fishcall posed a basic question); rather, pointed out that d.f. is freq. dependent & that if the B&K can hold df=100 across the band, it would be doing very nicely. Why 100? 'cuz Fishcall read that off his amp's spec sheet so I stuck with that #. Is it enough for his room, his music, his volume level? Don't know!

How much df is enough is very debatable & many view points exist. The Gamut website has a paper saying that df=25 is more than enough. This # falls in your 10-100 range but it is rather subjective whether df=25 will suffice. What I find when the df is on the lower side that such amps cannot be used in any system that one pleases i.e. the amp & speaker have to be carefully matched. I consider this to be a negative aspect as the user's favourite speaker might be precluded from the list. Others might not feel this way at all. However, I agree with your view point that too much of df can be detrimental. It seems easier to pick out the lower limit (where bass is wooly) & the upper limit (where the bass is dry) but I'm not sure that 10-100 is that range as I've not done any tests myself to quantify this range. I agree that each one of us needs to choose the level of DF & its impact on sound in their resp. system.