Reposting a comparison I made between these and the 558s. Overall these are a great upgrade to the 558 that I am enjoying even more now than I was when I wrote this.
I just upgraded from the 558 to the 58x. I have been using the 558 for the last 5 years.
-Bass is significantly stronger and cleaner. I have a Fiio e10k and the 58x has more bass than the 558 with the bass boost on without the reduction in clarity.
-Treble is more present and cleaner. I am hearing details in songs that I never noticed before. Sibilance is about the same or maybe slightly stronger than the 558. I am very sensitive to sibilance and it's not an issue.
-Much better overall clarity and instrument separation. I have noticed that dialogue in tv and movie scenes with lots of background noise is easier to understand even with louder bass noise in the background. I have also noticed that the direction of sounds seems easier to discern, especially sounds that are coming from behind you. Everything just seems cleaner and more distinct.
-Somewhat smaller soundstage. Honestly, it's not too much narrower and I don't even really notice it other than immediately after I switch from the 558. After a minute or two I get used to it and start appreciating the extra clarity. On the other hand, when I switch from the 58x to the 558 it just sounds hollow and lifeless in comparison. Overall I find myself appreciating the extra clarity within the soundstage more than inaccurate width of the 558.
-THE CLAMP. My only real complaint. I've bent the metal part of the headband and have been leaving the headphones stretching overnight on the box it came with for the last several nights. It's finally getting pretty comfortable, though the earpad material isn't quite as soft. If the 558 is a 10 on comfort, I'd say the 58x starts at a 3 and is at about an 8 right now. I think it will get up to a 9 after another night or two of stretching. To be fair, the 558 had quite a bit of clamp out of the box, but not this bad.
Overall, I'm really happy with the upgrade. It did sound a bit congested at first, but you quickly get used to the smaller soundstage. The one thing I haven't done yet is play games with it, so the smaller soundstage could be a problem there. Based on my experience with movies, the extra clarity and ability to pinpoint the direction of sounds will be more than worth the trade off.
Final note regarding source power: I use literally the exact same volume on my phone and e10k as I did with the 558. I honestly can't tell a difference in volume, so don't worry about the higher impedance.
Some updates since writing this comparison about a week ago:
-I prefer the 58x to the 558 for gaming due to the precise imaging and overall sound quality improvements.
-Comfort is steadily improving. I have stretched the 58x on the box it came in every night and will probably continue to do so for at least a few more days. Probably at a solid 8.5 right now.