I must justify my position by mentioning the now old Sennheiser HD265 closed-back 'phones, as well as the current 25-SP series. Both of these are - IN MY OPINION - significantly less coloured in the midrange, more revealing of the heart of the recording and production and with loads of balls and life - quite the opposite of the average HD600 installation.
At the time, the new generation of electrostatic 'phones like the Jeklins, early Stax, Wharfedale Isodynamics etc. really showed how coloured the older coil type headphones were, the Pro 4AA's sounding cupped-hands on cocals in comparison. In fact I remember the "budget" Pro-4AA (the K6 Red Devils?) sounded a bit better and was ever so slightly lighter too.
None of my comments above deny that the 4AA were beautifully built and the featured liquid-filled pads were innovative back then. I suspect they'd cost loads if made now.
I can't go on and I can't go back, I don't feel so matter of fact,
I tried and tried to make good sense, What's the good to try it all again?