Great guide, a few notes though:
- Impedance is Z, Resistance is R; they are different things. Source impedance is therefore Zout or Zsource.
- The damping factor section is largely marketing fluffies; for a variety of reasons including cable resistance, and the reactivity of loads (it will inherently vary with frequency). See here for more: http://www.butleraudio.com/damping1.php
- Not all amplifiers decrease their output as Z increases; some are invariant (or nearly invariant) - the WA6 is a good example (it uses a transformer to accomplish this feat).
- The actual power needs are dictated by sensitivity, as you note, but perhaps explain the log relationship between power and output. See here for more: http://trace.wisc.edu/docs/2004-About-dB/ Basically consider peaks and the dynamic nature of music instead of static demands; the easy way to do this is to figure out how dynamic the material you want to listen to really is, and then assume that is "top end" static demand and reference that to whatever output SPL you want. I think most people probably will target 60-70 dB but maybe I'm just weird like that (I get the sense I listen at VERY low levels compared to most people, based on the lengths I go to to reduce gain; perhaps I just have super efficient cans, I don't know). So if you're listening to modern pop, you probably only need 2-3 dB of DR, and that makes your continuous power requirements relatively high. If you're listening to old classical recordings, you might see 40 dB or more of DR (the highest I've ever documented is right around 40 dB, but I don't listen to much "very old" music and I hate doing vinyl restoration) - your continuous power requirements are very low, but the peak demands are going to be substantial. Especially if you want the continuous/base-line level at a "loud ish" point (like 70 dB).
- Line out is not standardized. Pro and consumer gear targets different references (http://www.gearslutz.com/board/so-much-gear-so-little-time/403493-4dbu-versus-10dbu.html), and consumer gear has a tendency to be all over the place beyond that (I've seen fs spec'd anywhere between 1V and 5V). Some line drivers can actually run some headphones, some cannot. I've never seen a solid rule of thumb here (and it is not load invariant).
Also in terms of "what you will hear" - it depends on the relationship between Zout and Zload and what the amplifier can actually do. If the amplifier can drive lots of volts into the load but has Zout (e.g. a receiver or otherwise "large" amplifier, you could probably even count some amplifiers like the Beyerdynamic A1 in here since Zout is around 100ohms) it will likely just mean attenuation or boost at various frequencies, if the amplifier cannot (e.g. "line out" on a soundcard), it will probably just mean roll-off. The former is perfectly okay, the later is not (the former means "coloration" or whatever else you like; the later is just things not behaving).
Overall I think this is a great guide!
And I agree with the "plug it in and listen" as well - just because you have something with high Zout doesn't mean it has to sound bad. I actually liked my HD 580s out of a receiver with 470ohm Zout more than any other amplifier I tried them on; it added "body" to the bottom end. So that's a preference. Of course there are also headphones with Zobels (or otherwise flat impedance curves), and that are very sensitive, that absolutely ignore this entire problem and sound the same out of anything (higher Zout just makes them quieter); they're relatively rare ime though (I can think of three Sony models, Grados, and Denons).
