Well, calling 600's balanced while they're almost like 6-7dB short in bass is actually an overstatement. Liking thin sounding gear ain't no crime, but come on. I'll be happy to see 600/650 cultism slowly disappearing, due to them being just insufficient. Actually, the bass fundamentals range are mostly missing in 600's.
Not to say that I'm not personally quite disappointed with how APM's measure on the couplers, yet...
... accounting for typical head vs head variation between people, I'd say that treble tuning being -5dB from the target is a very safe way to go, considering that the target group either doesn't care or doesn't know better, and you'd be risking putting them into shrillness region, if boosted towards Harman - even here at ASR the variance towards Harman is so big, that we can't consider crowd here being positive towards the curve - and that's okay. Being shrill might be worse than feeling "muffled" - especially if you're trying to beat XM4's & Bose NC700, both of which are either dark or very forward.
My first impression with (won - so hardly buyer's remorse & self-compensating the act of spending much money) QC35-II was that they are so full and unmuffled that I was lotta impressed. So much that I became disillusioned w/ razor-thin KNS8400s and pretty much anything bass-lacking.
Lack of body is mistaken for detailed and that's something that we all should fight.
I said the HD600 is
more balanced. I totally agree on bass, it's definitely required for an accurate approximation of the sound the mastering engineers heard in the studio (which should be the aim), and so shouldn't be skimped on. Basically, both the APM and HD600 are flawed headphones, but for most people, the APM will sound more flawed (even for all the Apple cultists out there, not that they'd ever admit it). The APM has a similar overall spectral tilt to the Sony XM3's, which is to say dark and muffled:
To illustrate my point further using objective metrics from Oratory's measurements, take a look at the steep downward slope of the linear regression line through the XM3's error response (upper curve, similar in tilt to the APM as seen above):
Compare with the HD600, which has an almost flat spectral tilt:
In numbers, that's a slope of around -2 for the XM3
according to AutoEQ's ranking table, among some of the worst in terms of that metric (again, which will be similar to the APM). Contrast that to a slope of ~0.1 for the HD600, with 0 being neutral overall spectral tilt. And it's this metric (the slope of the error response) that has the majority contribution to the headphone preference rating formula as determined in scientifically controlled, double-blind listening tests.
