I think the hard part about all this, is that different amps will sound different due to topology and the quality of the circuit design. These differences are often reduced down to "synergy" and are basically not measurable. This makes it hard to determine what about a specific amp makes it better with one pair of headphones or another. Power is a big factor, but definitely not the only one.
In general, once you have enough power to bring the amp up to painful levels, having additional headroom isn't necessarily going to get you better sound. Part of it is paranoia on our parts as customers (do I have enough?) and partly the manufacturers (you don't have enough!).
One thing to consider for orthodynamics is voltage swing, which isn't explicitly addressed in the simple formulas that I posted earlier. They do want a little more voltage than dynamics. This is where battery powered amps have an advantage since they can produce a lot more voltage. There is only so much you can get out of a USB port. According to the calculator I have, the base model GEEK should still be enough for the Mad Dogs. The base model is borderline for my HD800s and I am considering the next level for them, however I envision using my GEEK more with IEMs and portables that are much more sensitive and I might lose resolution trying to attenuate all that power.
If you really want to go down the rabbit hole, there is a thread on headfi about power and orthos with a lot of knowledgeable, respected members who have different opinions on the matter. You will come out of it more informed but possibly more confused
I like to have the right tool for the job and don't want to succumb to my fears of "do I have enough" so I post info so that people who honestly don't know can make informed decisions. I am not trying to convert anyone to my philosophy. More can be better, but "MOAR!" rarely is. Super Duper GEEK seems to be more MOAR! than more and the GEEK seems to be attracting a lot of people who are getting their first dac/amp.
If you aren't using sensitive headphones, particularly IEMs, a little more headroom won't hurt, though I really think you won't need it at all.
