I have the KSC35, KSC75 and KSC50 (the one before the KSC75) and also owned the PortaPro and SportaPro in the same period as when I owned the 35/50/75.
Don't care for all the other Koss HP's to be honest but admit I did not hear most of them and also not the ESP950
.
Price range differs a bit as well...
I started my search with the KTX-Pro1 which sounded boomy but got me curious about Koss portable HP's.
On some of those when you hold it against the light you can see a coating vaporized on the diaphragm and the plastic frame also differs from color.
I found the Sportapro to sound bad comparatively and mostly in the lows and top end.
Could not discern much difference beteen the 35 and 75 though many say the differences are noticeable.
production spread or other cause ?
For me the 35 and 75 worked 'best', followed by the 50 and PortaPro the SportaPro was sounding worst.
I did notice that if you press them against your ear with about the same 'force' they started to sound more similar though.
This prompted me to experiment a little.
Since they all (except the KSC50) have the same mechanmism for holding the earcups in place you can unclip them and mount them on another holder.
After swapping drivers around and some listening tests all on the KSC35 holder I could not tell the difference between the drivers and the 35 driver in the SportaPro frame sounded bad.
It was as if the drivers hadn't been switched at all.
For me it is quite clear the spread in SQ is not caused by different drivers and think they all are closely the same.
I couldn't tell any obvious differences between the coated and uncoated drivers personally but that might just be my unability or unwillingness to hear.
I could clearly hear differences between the frames though and think this is because of the differences in swivel action (SportaPro) and the pressure with which they 'seal' the ear.
PortaPro rests on the head KSC35 brace is 'fixed' by the compliance of the thin plastic ring and the 50/75 have more comfortable and 'adjustable' silicone clamps with metal wire in it.
Also ear shape has a LOT of influence on the sound.
The KSC50 was too heavy in weight and 'hung' too much on the ears, also the ear clamp was too thick and less comfortable as the KSC35 and KSC75.
These differences are MUCH bigger than the (to me marginal) differences between the coated and uncoated drivers.
I like those headphones certainly when you do not copmpare them directly with better ones.
To me they are very 'convincing' eventhough they have their deficiences they do not seem to bother me as much as other headphones can.
I can wear them for many hours in a row.