Synergy is an important factor, and I never said otherwise. Output impedance of an amp and the impedance curve of a headphone are a big part of this that many weren't aware of until recently (including myself), but that's still just one part of it all.
Also never said you can't enjoy what you enjoy, but the Grado headphones are pretty bad on most objective levels, and the fact they all measure so similarly across the product line is interesting to say the least.
Not all measurements are done the same way as well. Some are done better than others. I think the ones at Innerfidelity are good, but I'm not crazy about the compensation curve used, and I don't think they don't show differences between headphones as well as some other places do. They might also not specify something like which pads are used.
I've gone more to changstar.com lately for measurements. You can even see that the Grados on there measure fairly similarly (harder to find since the manufacturer is not listed), but they do have enough slight differences to give them unique characteristics. You have to take into account how frequency response, distortion, spectrum decay, wave response, etc. all affect what we hear.
I'm sure you will agree that Grado headphones do have a particular sound to them across the line. That is evident in measurements no matter where you look, and it seems to be the general consensus online as well (as trustworthy as that is...). They also have unique characteristics per headphone, which are noticeable when compared to other headphones in the line but less noticeable when compared to headphones that sound dramatically different. Again, measurements show this.
On the contrary to your point, I have heard many headphones that measure similarly and sound similar. I have also heard the opposite. You can even run into situations where QC is so poor at companies (Beyer, lately) that even the same model of headphone can sound different from unit to unit.
Correlating measurements to subjective impressions takes a lot of understanding and a bit of art (we don't, after all, know EVERYTHING about measuring headphones, but do know quite a bit). One cannot just look at a measurement of a headphone and be done with it...it's good to cross compare to see results from different measurement techniques, as I touched on earlier.
Unfortunately, not all of us have the opportunity to test out headphones easily, and they can cost a lot of money. It's pretty apparent that the audio industry is full of snake oil, myths/lies, and a whole lot of "audiophiles" that don't have a lick of objectivity in their bones (or are too objective and don't understand the subjective side). What, then, do I use as a reference point to make buying decisions? The vast majority of subjective impressions for audio gear tend to be noise from people that are more interested in feeling good about their purchase than actually being critical about the products on the market and not accepting under-engineered crap (most of the headphone industry).
Measurements aren't the end-all, be-all, but they serve a very useful function. It just takes a lot of time and learning to really understand all aspects of measurements, how those aspects contribute to the sound we hear, understand how different sources have different measurement methodologies and which of those best correlates to your hearing, and so on.
I get the appeal of Grado, and it's fine if someone likes that sound. It was the first major headphone I owned, and I loved it for a while. But they are just not up to par on any objective front, and they really started to fatigue me after a while. Now that I'm used to smoother, cleaner, and more neutral headphones, trying to switch back to something is nearly impossible. It's easier once your brain burns into the sound (let's not even get started about how often our brains fool us).
Someone's got to step in and make recommendations based on a reference point so that they can get a better picture of the headphone market landscape, which also helps when others are making subjective recommendations (easier to figure out their tastes and point of reference).