I owned both. Still have HD555, but sold the HD380 Pros. They sound very different from each other. HD555 is definitely much more fun sounding than HD380, with a much warmer overall tone and much better extended and sparklier treble. Compared to HD555, HD380s sound dry, somewhat thin due to understated mid/upper bass, and very muted on the top with pretty much zero sparkle. Due to lack of bass and treble, HD380s also appear to have an overly aggressive midrange, especially the upper mids to my ears. Music with a lot of vocals and instruments that mostly lie in the mids often sounds too forced, too shouty. HD555, on the other hand, has a much more balanced and polite presentation overall - the bass and treble are nicely present with sufficient detail and the mids sound about right, although maybe recessed sometimes. Also, HD555s also sound much smoother than HD380s, which tend to sound rather grainy and coarse in comparison. In terms of soundstage, HD380s have an impressively wide soundstage for closed headphones, but they lack depth. HD555 have more depth, but sound a bit narrow and closed in in the mids by comparison, although, as I already mentioned, their much better extended, delicate and airy treble really help HD555s to sound more open. In terms of imaging, the HD380s again provide impressively wide separation, but HD555s sound more focused IMO and less blurry, although amping HD380s, at least with a good amp, does seem to cure the graininess. One thing that I really like about HD380s, apart from their wide soundstage, is the detail that they produce in the bass and mids, especially mids. The detail level in those parts of the frequency spectrum is definitely higher than on HD555s, but unfortunately for me all that detail is completely compromised by HD380s lack of any decent treble response - they just seem to roll off completely after 10-12 khZ or so.
Now, in terms of comfort, both are quite comfy IMO, although HD380s' have cheap pleather pads that will make your ears sweat more especially in hot weather. I tried swapping the HD380 pads with HD555 velour pads and although the velour pads from HD555 fit the HD380s perfectly since the cups on both 'phones are the same size, unfortunately they really degrade the sound quality of HD380s making them sound much thinner with very weak, almost non-existent bass and virtually no isolation. Thus, I had to swap back to the original pleather pads, which give good comfort, good sound and pretty decent isolation that's about on par with that of most other closed type headphones, maybe a bit above average.
All in all, despite the fact that I really did not like the sound of HD380s, I still respect their technical abilities, such as great detail retrieval in the bass and mids and great soundstage width and dynamics and I think that for their price, they are still a pretty decent choice for use in studios for music recording, if treble is not that important. For personal listening and music enjoyment however, they are absolutely no good IMO. They don't bring music alive in the same way as HD555s and other decent audiophile cans do due to the severe lack of treble and understated bass. So yeah, I think you will like HD555s much more for music enjoyment, although you will miss some of the fine detail that HD380s are able to retrieve. Personally, for a real upgrade from HD380Pros, I would get a higher grade headphone, like HD600, or ATH-AD900 since it will sound more musical and extended in the highs that HD380s while also providing you with at least similar level of details and soundstage width.
Hope this helps.
