The HD58X does NOT use 5 series drivers. The drivers stem from the HD700 - just like the ones that sit in the 660. They are not precisely the same but it’s clear they come from the same family. Look up a frequency response of both and it becomes fairly evident as the two headphones look almost identical from the mids and up through the treble. Same damping factor and sensitivity as well. Think the difference between HD600 and HD650.
The 58X accomplishes this feat of having somewhat pricey drivers inside a tremendously cheap headphone by skipping a couple of steps in the production line…aka there is no view of the drivers. Instead of having to pretty up the drivers Sennheiser threw some acoustically transparent foam in the cups…and we ended up with yet another 6 series headphone..somewhat in disguise.
‘The headphone says 58X!!!! Must be related to the 5 series then’
Well it belongs about as much in the 5 series as the classic 580 Precision does
To me it sounds like a more exciting Sennheiser with better and tighter bass quality and reach down low. Like the 660 it rolls back the uppermids ever so slightly and gives off the same ‘exciting’ treble performance…but as mentioned earlier: the 58X is a tad warmer in the bass, and at least to me, sounds better and more inviting than it’s daddyo. Both headphones share the same edgy treble..but it’s only over the 660 that it gets hot and irritating for me.
The 58X is like a more refined ZMF Aeolus. I’ve a/b’ed the two quite extensively (my old friend owns the Aeolus) and find the Sennheiser to be the better headphone. They are both tuned with warmth and linearity in mind but it’s the cheapo Drop kid that sounds the most realistic and true to life per my ears.