Overall the sound sticks to that warm, neutralish tone that the rest of the line is famous for.
The big change is that the sub bass actually extends quite well compared to many open headphones, rolling off around 50-60 Hz rather than the 80-100 Hz rolloff of the others. This works well with all genres, but should be welcome to EDM, Hip-hop, and other electronic music fans who missed that in other HD6X0 headphones. At times there can be a little bloom at 125-200 Hz depending on the mastering of a given track, but bass remains well controlled and surprisingly detailed for a headphone in this series.
Midrange is very similar to the rest of the line with the upper mids (1.5-3 kHz) being slightly more laid back than the 600 and 650. Detail is very good and distortion is very low, as one would expect.
Treble is slightly more aggressive than the “veiled”, as some would call it, classic HD6X0 sound. There’s not much problem with sibilance to speak of as the 4-7k area feels a bit more laid back. I do hear a slight peak around 9-10k.
As far as imaging goes, these headphones surprised me. Very good sense of space and separation between instruments. Less congested feeling than I would have thought given my experience with other HD6X0 headphones, though not a match for the likes of higher tiered headphones like the HD800.
The 58XJ is immediately more extended in the bass and sounds a bit more detailed there to my ear. The 600 is cleaner and thinner sounding with a more natural timbral quality to it, though the 58X isn’t far off by any means. Timbre is the one thing the 600 does better than almost any headphone I’ve heard. Upper mids are more forward on the 600 which gives more of a sense of detail and clarity, though I would say in overall resolution the two are very close. Imaging on the 58X is much less “3 blob” than the 600 and feels more accurate to my ears.
The 58X is cleaner and better extended in the low end. It isn’t as thick in the mid bass and low mids as the 650, though retains some of that warmth the 650 is famous for. The 650 again seems slightly more detailed, but again, I’m not certain that’s down to sound signature or driver capability.
Take this section with a grain of salt since I’ve spent very little time with the 660S and haven’t compared these two directly. Overall tonality is very similar with the 660S, which feels more thin and more resolving than the 58X. The 58X has more and deeper bass. Like I said though, I’m going from memory here, which isn’t probably the best metric.
I’m really quite pleasantly surprised by these headphones. It truly takes something from every member of the HD6X0 family and puts it to use in a nice way. It’s not as clear and perfect in timbre as the HD600, but has better imaging and less "shout" in the upper mids as some don't like. It’s not as warm and smooth and romantic as the 650, but it’s more exciting as well as cleaner and deeper in the low end. It’s not as resolving as the 660S, but it’s immensely better value and shares many similar qualities. You can hear a bit of everything in them as well as its own strengths. They’re a great jack of all trade headphone and for $150 I absolutely can’t think of anything better. If you see them on the used market or they drop again in the future, don’t hesitate to at least give them a look.