Quiet day in the shop…so more time to voice my opinions/observations. (keep in mind I'm going from memory , but I had the HD580/600 for almost 10 years and sold them about six month after getting the 700s in Jan 2014)
Over a six-month comparison with my equipment and ears the HD700s continually proved they have way more base definition, depth and extension over the HD600s, if that’s perceived as distortion…so be it?
As for ringing...the 600s are too laid back for it to be an issue while the extended higher frequency energy of the 700s have been no problem what so ever.
The thing to keep in mind is the resolution of higher harmonic overtones off the fundamental which diminish greatly, these are almost none existent with the 600/650s but not with the HD700s. Also where as the frequency is measured rather flat from the HD600s relative to the base and treble output…Sennheiser have voiced the HD700s to psycho-acoustically reproduce IMO a more natural sound stage, the 600/650s producing a bigger than life central image and what most are accustomed to with headphones.
So keeping that in mind… microphone measurements to emulate true world performance should only be taken, not with a generated tone above musical fundamentals but only of the generated upper harmonics through narrow band equalization of the fundamental generated tone, which are generally below 4k Hz otherwise due to voicing the higher frequency output of the headphones will be greatly over exaggerated and may be perceived as peaks.
So why do some hear peaks considering the amount of aural information which actually encompasses the upper-harmonics which should be in real life greatly reduced due to aural decay as to be non offensive…good question. Well power of suggestion is an extremely powerful tool, if I tell you such and such a headphone has undesirable peaks (when in reality there are none) one will still most likely experience them…but more so a lot of recordings today over emphasize treble information, compounded by the fact that higher frequencies are very difficult to reproduce digitally due to intermodulation distortion which are compounded by being recreated by mediocre DACs and most importantly the analogue output stage which gets amplified by the headphone amp which in turn can create/add more intermodulation distortion which the HD700s are more than happy to reproduce.
Sennheiser would love to sell boat loads of HD700s but the truth is that I sincerely believe that the less revealing HD598s may be better suited/rewarding for the majority of users, the HD700s are IMHO a much better headphone but as I’m seeing not for everyone but if ones willing to take the plunge do so.... with an an open mind…. approach things with an unbiased objective opinion.