I wonder how much of the HD5x8's harmonic distortion is caused by the enclosure and overall implementation (ex: fabric behind grill vs. entirely open grill, foam directly behind driver or not, etc.) and how much is simply inherent to the driver.

I don't doubt the HD6x0 drivers have inherently better performance, though I do admit I was surprised how close your HD558 and HD600 measurements were a few posts back.

Do you have those re-done distortion measurements to compare the headphones?

I don't doubt the HD6x0 drivers have inherently better performance, though I do admit I was surprised how close your HD558 and HD600 measurements were a few posts back.

Do you have those re-done distortion measurements to compare the headphones?

Yes, to my ears the HD600 and the HD558 seem to be pretty close. That said, I feel the HD600 is a tad less dark and cleaner sounding.

Below are the distortion measurements of the HD558 and HD518 right drivers using 90 dB SPL calibrated both with pink noise.

HD518



HD558



Distortion numbers are not exactly the same, but they are not so grossly off IMO.

so hands down do you like these much better than the m50 Ultrabike? I was thinking of picking up a pair myself.

Not necessarily. I prefer the HD518 to the M50 in general. But the friend that lent me his HD518s also lent me his M50 both which I measured, and he prefers his M50 hands down.

He told me that the measurements did correlate with what he heard. He is also a fairly proud bass head, and likes what his M50s bring to the table. To him the M50 have cleaner bass, and I sort of agree with that. He is more into speakers though, and my previous comment about him looking into a DIY > 18" sub is a fact. He has a HSU 15" which makes his pants happily flap, but wants moar.

In regards to the HD518 vs HD558, the HD518 extra upper bass does has it's appeal to me. In fact, I could easily fall for either one depending on my mood and universe alignment.

All of these are good cans IMO.

All the HD500 and 600 series roll-off in bass far earlier than what the graphs here would indicate.  That's my biggest beef with them.  Only the HD800 has some decent extension, but I don't like the overall balance of its signature.

So yea that's my reason for not choosing an HD5-6xx.

I think my HD600s fit me a little tighter and better than my HD558s, and that might account for some of the perception differences w most of the music I have. Perhaps that's why I felt they were "cleaner" in the bass area, even though the measurements do seem to indicate lower distortion all the way to the upper bass (hearable or not).

Other than that, higher impedance is actually one of the main reasons I finally decided to get my HD600s: Flexibility to check out other amps out there.

As far as there being other better dynamics (orthos or stats), that IMO is a question of priorities, personal preferences, and chaotic mood swings. Most of the time though I do seem to gravitate more towards the signature of the Senns than most other cans (including Stax and Audeze). No immediate plans to sell either of my HD558s or HD600s.