I did not know the HD555s changed impedance over the years. When did all of this happen. I bought mine apox. 7 years ago. I used them for a long time, got into my speakers system.
(NAD c160, restored NAD 2255 non power envelope poweramp... the last 3020 based amp they made, PC built exclusively for silence, but also the power to do my CAD work, storage, and game, and PSB Alpha Bs for nearfield, and PSB towers for HT use, along with a selection of nearfield studio monitors, vintage sansui tuner and my MHDT DACs as well an external tube buffer for my smaller MHDT, as my other is newer with it's own tubed output stage.)
Anyway, for personal reasons, I needed to move the hub of my work/media into another room, use a laptop for work and remoting into my other PC, and put together a 2nd system for use with headphones (external usb s/pdif from the laptop, playing files over the network), thus the addiction began, and now I own a variety of midrange to upper midrange (Beyer DT880s are probably my best, but I have some good closed backs too plus a pair of Grado sr325s, the originals).
I dusted off the HD555s one day and, knowing about the mods, I went ahead and removed the foam, along with a grillecloth just behind the wire mesh. I have not removed the plastic grille because it allows so much airflow that removing it would not do much. I noticed that most of the assembly is plastic click locks/tabs, and metal tabs bent to hold. I had to add mass to the drivers with blue-tac and such, because these really do move a ton of air, and that foam you remove protects the drivers from hitting the inside grille. The drivers state 55 ohms, so I am assuming mine are the higher impedance; 55 X 2 + overhead = 120ohm versions? This has me curious because I was not aware the HD555 had multiple versions over the years.
Everyone is correct, they require very little power, the mods, I did at work on my lunch hour, and finished up at home, since I had to adjust the cable and repair some wear on the pads, all of that needed a good locktite adhesive. I use something that sets like a rubber cement, but seems far stronger and very useful due to the speed it sets at. I would never use superglue (resin) for anything like this, could hurt the plastic and such, and it oxidizes too much to hold well in this type of repair. If you get new ones, you are looking at an hour or two of very easy work. It is one of the easiest mods that gives the most results out of almost any headphone out there. If you do it right and work at it, they will surpass stock 595s, and come very close to HD600s. The only thing holding them back is all the plastic locks and tabs, because the HD555 was not designed to be an HD600, obviously, so the driver assembly is going to be the biggest difference.
After spending a lot of time listening to my other phones, I pulled out the HD555s and did the mods, which is strange because it took me a long time to actually go back to them and give them a shot. I was into this hobby and my interests were in my other headphones. I was blown away. These things just came to life. They have their flaws, but damn they are fun to listen to. I am not going to replace my DT880s with these as a critical listening phone, but I put the Grados away, as those needed a lot of work to sound right and these can plug into almost anything. They are the open version of something like an m50, essentially a damn good phone for a damn good price. The 595s are not bad at all, and supassing them makes your modded HD555s nothing to laugh at. People will dump on the 555/595, but people dump on almost every headphone. Look at the HD800: one of the best dynamic headphones made, but they certainly aren't for everyone (If could afford it, I would have a pair of Beyer T1s on my ears right now).
Seeing as there are relatively few full sized cans that can be driven by a little mp3 player, I would place the 555s high on my list to recommend if you plan to do any kind of listening from a source and no dedicated headphone amp. Even good portable headphone amps are not meant to drive the big boys, but just improve the cans that can already be driven by the portable player.
