Electrolite
100+ Head-Fier
58X FR is closer to 660S in the upper mids - less aggressive there than the 600 and 650, but it does have the most bass out of any of these yet I found it less controlled than the others. Driver source is still total speculation. I have no idea what they're really using in there, but it definitely reminds me of the way the 595 sounded in terms of grain but with a 6x0 style FR. I was probably too harsh on it in my review as I somehow forgot that a lot of people buying this thing are doing it as a first ever purchase into the hobby, and in that case being easy to drive is of paramount importance. But I still strongly feel that the 6xx has more resolution yet not as pleasing an FR, with more upper mid shout a la the classic HD 600 and 650 sound, and you'd want to use an amp for it. FWIW I now have both a 6xx and a 660S to reference other headphones against.
So you think the 58X is a little more pleasing? I thought it was the livelier one.
Anyway the 58X has the best bass extension, a little more pleasing FR compared to 650 and it's portable, don't you think that's worth to lose some detail as a trade? You gain some good benefits, I thought you were a little to hard on them, unless this detail loss is very significant, but if detail loss was ever mentioned most reviews said the difference is small compared to the 600 and 650.
For me the 58X was boomy lacking some bass control and grainy but with time as pads got older both issues went away, I also started using Oratory EQ and put some felt in the driver. Not saying something like wooow it has much more detail and is superior now but all of this helped to fix many of the 58X flaws.