Oratory1990, rtings, Crinacle and Resolve have each published their measurements of the 660S2, and they tend to differ in important ways. But they do agree on the bass and low-mid, and my ear agrees with them about that range. There can be some variation in the sub-bass, but that's easy to tune by ear. The rest of the spectrum, I've decided, is probably too subject to unit variation and measurement variation to continue using even my own by-ear variations or attempted integrations of published EQ recommendations. I do not know whether this amount of variation is normal for the industry, or if I'm just being overly-picky about what I'm hearing and not hearing.

I had two beginning ideas:

Perfection. When it's perfect, I can use the green mod, close my eyes, and see the full band in perfect detail. Every instrument and vocalist occupies a precise place in 4D space. I had got there with my Aryas, but they were too big, and the pads were shedding terribly. And, you know, it was time to sell the luxury items. Anyway, If the snare drum occupies a pin point in 4D space, the rest of the band should fall into place on a good recording.

Parsimony. I always make the fewest and smallest changes necessary to get it as close to perfect as possible, but avoid spikey Qs. If I can barely tell the difference a filter makes, or if I'm not sure if it's better with or without it, then I don't use it. I shoot for a fairly flat bass line from 100Hz down to 30Hz, preferably with small a dip centered around 80Hz to keep it sounding tight and clean. Preamp eats dynamic range, and the quietest parts of the sound are what give it realism. And I want to keep noise as low as possible. So when possible, I cut rather than boost.

I am really happy with what I've got, so far. I boosted the bass, brought the mid forward, and brightened up the upper mid. I saw a video on Youtube that told me how to get the data from the rtings chart in a form that REW can read, and used that to adjust the bass and low mid area. I tried to bring 55Hz up to the level of the mids, because that's the kick drum. After boosting the bass and low mid, I boosted the upper mid around 2700Hz to try to make it sound more like the 600. I wanted to boost that region without making it too fatiguing, and avoiding the much-discussed bottomless pit around 4500Hz. This made the mid sound hollow, so I brought 650Hz up a bit, too. I played around with closing my eyes and slowly adjusting the slider with the scroll wheel on my mouse, and got it as close as I could get it. I ended up boosting bass, boosting mid, and boosting upper-mid. Shouldn't I just cut everything above upper-mid, then? I tried it, and it was worse than no EQ. I guess, to my ear, the 660S2 has an M shape, so I gave it a W to flatten it out somewhere louder than the treble.

It's not quite like being there, but it sounds amazing-- especially for under $500 (bought on sale). I really don't know what my endgame would be. The next step that makes sense for a fanboy like me would be the HD800S, of course. Everyone says it takes EQ really, really well. But they also say it's for people with really large heads, and I have a small head. And its price keeps going up, while Hifiman's prices keep coming down. I fear the build quality of Hifiman and Focal. Maybe Meze Elite or Empyrean? I just want to be able to believe that I'm in some huge room with the band right in front of me. I'm willing to save up a while. The 660S2 should keep me satisfied and tweaking EQ for a few years, at least.