OK, I just went over Headroom's explanation of how to read the charts again, and double checked all the charts (square wave, sine wave, frequency response...etc), and all the candidates performed quite well in all the other categories, and only really differ in frequency response. So I made a screencap of their FR and then in Photoshop drew a white FR graph that reflects the "ideal" FR according to Headroom's description of what the most natural sound should look like (3~4db hump in the 40Hz~500Hz range, gentle slope of -8~10db starting from 1KHz to 20Khz, and preferably with some small dips in the 2Khz~8Khz range). So, if the perfect headphone existed, it should look very similar to the white line I drew:
(The black lines are markers for the points where the humps and slops and dips should start/end.)
So I studied that chart and then also compared it to the FR graph of the headphones I own or have owned and know well, to extrapolate reference information based on past experiences and how they relate to the graphs. My conclusion right now is that the K701 should be eliminated due to the unnatural hump in the 6~9Khz region, and the fact that it does not start gently sloping down at 1Khz. I think that explains the often complained "too bright" sound. It's upper bass isn't all that great either, and doesn't extend as low as the 601 (which on paper looks better than the 701).
So now it's down to the HD600, 650, and K601.
The K601 doesn't bump as high in the 40Hz~500Hz region as it ideally should, but it extends really low (-2db at 20Hz, which is remarkably deep), and it's treble slope actually looks pretty smooth, although the highest top end sharply drops off, so the most delicate sizzle won't be there (and probably won't be missed by most adults with average hearing ability anyway).
The HD600 has almost the classic ideal in the bass region, and it's treble region averages out to be really good--almost ideal also. No wonder many consider it the ideal classic standard in which to judge others by.
The HD650's bass is definitely humped more than ideal and extends very deep like the 601. I like that it extends very deep, but I'm not sure how that extra hump above the ideal 3~4db sounds like since I have never heard the 650. The treble looks fine to me, and has that sharp drop off just like the 601, which I guess explains why some people feel it's a bit too laid back and warm.
(BTW, when I looked at the HD800's graph, I can see why the guys at Headroom has crowned it as the best--it's got really amazing ideal FR. But it's way over my budget.)
So, I have to choose among the HD600, 650, and K601. Any more advice?