Before buying the 58X, I was not in way a Sennheiser fan. Most all I had experienced were to my ears veiled, congested and bloated sounding. After taking a $160.00 chance on the 58X that has changed. Truth is, if I wasn't so impressed by the HD 58X performance I'd likely never tried the 660 S. After listening with both, below are some personal observations.
The Devil's in the details! The HD 58X is not veiled, but it simply doesn't have the detail retrieval of the 660 S. The bass is by comparison exaggerated on the 58X, but it is not bloated. Both perform best with a good amp. Neither of these cans become harsh with a decent recording. Some have mentioned a grainy sound, it doesn't seem to be present in either of mine. The imaging and soundstage on the 660 S seems noticeably better than on the 58X.
I've twice tried the grill foam removal on the 58X and honestly can detect no notable difference. Years ago with the 500 series there was a noticeable change when removing the thicker mat inside their grill. (Either way they still sounded like hell to me)
As I type this I'm listening with the 58X and enjoying it immensely.
I don't own anything below $300.00 that I enjoy as much as the 58X.
The build quality is I believe equal. Other than the obvious grill logo and shiny headband from a distance they look pretty much like the same cans.
Is the HD 660 S better at clearly delivering the majority of what I listen to? Yes.
Is the HD 660 S worth 3 times the 58X cost? For many if not most, probably not.
I like the 660 S performance enough for "me" to justify the difference in cost. I did wait till I found them discounted to nearer $400.00 before purchasing. I'm still quite pleased with 58X. They are a genuine bargain!
If you have a decent source, good source material to feed them and you're stuck on the bubble about buying these, at $160.00 they're a steal.