slew rate and rise time

'Z' means impedance in 'electronic speak'

Amps damping factor is another one of those minimal specs.
Above a certain minimal value they don't mean much except to the advertising copywriting guy.
Besides, the amp doesn't 'absorb' or 'Damp' or 'control' the Back EMF from the woofer. Rather, the woofer controls itself.
You can test this idea by thumping the edge of a woofer with a finger. Than, short out the leads to the driver with a piece of wire. Thump again and the sound changes lots. this is the woofer damping itself. Low input impedence of an amps outputs just allow the current to pass thru, back to the woofer.
More (arguable) is the speaker systems design 'Q', which also is an expression of damping. What is called 'critically damped' design of Q=.7 will give nice tight bass even with a minimal damping factor amp. A speaker with a hi 'Q', say 1.2 or so, will sound sloppy, even with a high damping factor amp.

Jj2468 & Rleff : In general, the specs for output Z will be listed in the owners manual of the amplifier or on the manufacturer’s web site. One should also note the input Z as this can come into play when interfacing a preamp or source to the amplifier.

As to damping factor, a damping factor of 200 or 2000 will be indistinguishable in practice. The damping factor is just Zspeaker / Zout amp. One could say that this is equivalent to specifying Zout of the amplifier, and, technically that is true. The problem that I have with paying a lot of attention to the damping factor is that it is a step removed from the fundamental parameter, the output Z of the amp. One must keep in mind a caveat - the damping factor has to be stated with regard to a given load impedance, usually this is 8 ohms. But, no “speaker” has a constant impedance from 20Hz to 20 kHz. In deciding whether the changes in speaker impedance over frequency will affect the sound, it is simpler and more straightforward to look at speaker impedance curve or the minimum and maximum speaker impedance specs and then compare those to the amplifier output Z. So long as the ratio is better than 100 or so, you should be fine. Whether it is 200 or 2000 is not going to matter. When it comes to figuring out how to select components for a system and what could cause problems in a system, thinking in terms of base parameters rather than derived parameters makes life easier because it lessens the tendency to unnecessarily complicate the the issue.

Damping is very important but it is more related to speaker design than anything else. It is determined by Q factor. Above 10 you start to get diminshing returns on an amp - above 50 you are more less getting close to inaudible improvements in damping from going higher. A speaker will have a Q of .707 to be critically damped. Many speakers have a bump in the bass and are underdamped - no amplifier can compensate for the sloppy resonant sound of this type speaker.

Rleff - Keep in mind what "damping" is, as opposed to "damping factor". If you look at the response of a system, any system, to an input signal, the question is how well does the output follow that input - both in terms of time and magnitude. At the extremes systems that are not properly damped are will either be too slow or will have too much overshoot. Damping can be accomplished by mechanical, electrical, pneumatic, etc, methods. Damping factor on the other hand, at least in terms of audio amplifiers (as opposed to the definition found in control system theory) is simply a ratio of impedances and at least one of the impedences is never fixed.

Jj2468 - in response to the other half of your question - a low output impedance will be somewhere less than 0.04 ohms. Most solid state amps will be there without a problem. You are more likely to have high output impedances with tube amps.

Shadorne, At least you get it.
Once above a certain minimal value, who cares?
The Speaker has the last word here. I have heard a speaker alledged to have a 'Q' of .707 and it sounded almost bass thin.
Does a low 'Q' speaker have what I have heard referred to as 'bloom'?