I recently purchased a pair of Senn HD-555. They were in absolute mint condition, haven't been used much - also they have had the "595 mod" done by the previous owner as I have opened them both and checked.
I am not very pleased with the overall sound. I am no audio geek and my sound "rigs" consist of either an iPhone 4 ( which in my opinion has an incredible sound quality output ) or a home PC with Fortissimo 4 sound card + Pioneer AV Reciever ( VSX-405RDS MK II ). I find the sound on the 555s to be very neutral, with kind a dull base response and not so clear highs. I also own a pair of AKG K181 DJ to compare them with. While the level of detail is fine on the Senns, \ I find the AKGs to be better in every other aspect : punchier cleaner bass and much more crispier highs and vocals. Also the IEMs I'm using ( Brainwavz ProAlpha ) seem to take on the Senns on every level - although I'm not sure an open headphones vs IEM is quite a good comparing.
I don't want to over crowd this thread as I already have opened another one regarding an USB DAC/Amp and I have more details there:
Will the 555s feel any different with an amp as my home PC source is not that great when it comes to SQ and the iPhone doesn't seem to be very comfortable driving the 555s ? Or this is the "normal" sound of these 5xx Senns : very neutral/dull.
I do WANT to love the Senns as they feel absolutely superb on my head even after hours of use... but... :)