Without doubt, there have been some excellent cans from 20+ years back (if memory serves me, the Senn HD-580/Precision a fine example).
Yet, there is an endless array/combination of not only drive-units (build quality/consistency) but chassis considerations; mounting techniques, driver angle, material selection and headband (and ear-pad) design (material, shape) but a few considerations. Although it's not rocket science, Sennheiser in particular appears to veer from tried/true design criteria with respect to tonal considerations, and basic head-band design/material choices; the single piece (L-to-R) foam headband 'cushion' (that's directly attached to the frame) is an example of backwards thinking -and questionable merit. (Think the head-band consistency of Stax, Beyer, AKG, Hifiman)
It seems Sennheiser (as of late) are more interested in aesthetic appeal as opposed to tried/true design integrity that was historically a given with Senn's. The sheer number of models available within the same mid-level ($200-$500.) price range is somewhat dubious. If for example, Senn had a Group(ing)/skew that separated/distinguished (made sense of) the in-numerous models on offer, personal selection would be far less daunting.
As it is, I'm left to assume the company is throwing (similarly priced) models out there randomly hoping one "takes". Of course, in the entry/life-style category (shaped by flavor-of-the-month buyers), that's understandable. However, in the 'audiophile' category, the barrage of 500-600 series models, while the not-terribly-impressive (and expensive) HD-800 has been relegated to the forgettable-bin by the 800S that by all rights should have been given a new model-series designation (such as "900" ,HD-810/880 etc.) given its sky-high price (jump) compared to the original -already an expensive headphone. By many accounts, the 800S is far superior to the standard 800; its Star-Treky styling ear cup (and headband) design appears to be the 'star of the show' as opposed to sound quality; elevating the resolution, transparency and accuracy as would be expected at this price point.
Right then, not sure how I veered off topic here other than to say I'll pay homage to classic Sennheiser by ordering Massdrop's HD-600/580 clone at $149. Now that's a price/performance recipe easy to digest -even if its performance falls short of the exemplary Hifiman 400i's.
pj