Having owned both, I feel that the 58X Jubilee has a few positive qualities over the 6XX. But i ordered both of them about the same time, the 58X just arrived first.
I've since given both away as gifts. One to my stepfather (who's a recording engineer) and one to my mother (who just wanted nice headphones).
The 6XX is nice, to my ears, it's the deeper headphone. It draws your attention to parts of the music that you might not normally notice. It's not fine treble detail though... Though the 6XX is plenty resolving imo.
The 58X has more treble energy comparatively, and doesn't scale like the 6XX does. However, it's easier to drive, and it's lower cost made it easier for me to bring to work and beat on. Plus, I worried less about doing mods. With the 6XX I was too scared of ruining the sound to change the pads. Measurements and anecdotal experience says don't do it. But the 58X? I swapped them Dekoni ELVLs on it, and it seemed to eat some of the treble energy while making the comfort soooo much better.
Plus, the 58X just sounds great in general. I've never hesitated to put it on someone's head to hear what great headphones are. While there isn't as much bass impact on the 58X as the 6XX to my ears, I do feel that the 58X has a more controlled low-end. Similar to the 600 but more present? The 600 is amazingly clear low end, but less impactful than the 6XX or the 58X. Beyond that, I feel that the midrange performance is similar on all of them. The 6XX sounds the sweetest, I think. But the 58X and 600 sound just, really really good and natural. The 58X might be a half step less intimate than the 600 though.
I really enjoyed the 58X and I spent a lot of time listening to it, before I passed it to my mother who listens on her iPhone with the apple Dongle. On that, it sounds good. With the Dekoni pads, it's super comfortable as well. I mean, this is subjective but as someone who owned both, I don't regret owning the 58X and the 6XX at the same time. I think they're both fine headphones, and if you're curious, and can afford a pair, buy one. Or see if a friend will lend you one to test for a few weeks.
I don't know your ears though, and some prefer the 6XX or the 600. The good news is that because it's a decently popular MD item, you will probably be able to sell it quick if you don't like it! Good luck!
Side note: for anyone who wants to know what tracks I generally test with and what my upstream gear was for testing, I've used them all with the SDAC+O2 combo unit, Aune Stack, and Fiio Q1 Mark II. With plenty of Flac files. A few of my favorite songs/albums to demo are the Overture from the Halo 3 ODST Soundtrack, Mountains from the Interstellar OST, the whole of Run the Jewels 3, Bayani and Cinemetropolis for bass, as well as some Yosi Horikawa's Letter for imaging. Also, The Little Light of Mine, from Chesky's the Ultimate Headphone Demonstration Disk, and Grandma's Hands from Chesky'sThe Ultimate
Demonstration Disk for vocal prowess and timbre. As well some Binaural Demo Tracks from HD Tracks which I can't remember offhand.