Also note that the Stax SR-007 Mk 1's sound different from the Mk 2's and quite different from the Mk 2.5's (Mk 2.5 SR-007's can be identified by a serial number starting in 'SZ3xxxx')
I'd say that the ESP-950 shares more sonic characteristics with the SR-007 than it does with other Stax headphones. The Lamda's have a bit of treble emphasis that distinguishes them from the ESP-950's which have more neutrality in their treble range. The SR-007's don't have treble emphasis either. So in that sense, both the SR-007 and the ESP-950 give you a good midrange without being "overbalanced" by treble emphasis. Both the SR-007 and the ESP-950 have extended and very capable treble, but there is not a treble emphasis like many other headphones. But they don't sound dark, either.
I'd say the main differences between ESP-950 and SR-007 are these:
- SR-007 is quite neutral in character, with perhaps just a little emphasis on a warm, liquid midrange. The ESP-950, on the other hand, has a bit of upper midrange emphasis, giving them a slight forward quality. Also the treble in the SR-007 is 'smoother' sounding than the ESP-950. The ESP-950 doesn't have treble emphasis per se, to my ears, but does not sound quite as neutral as the SR-007.
- Koss ESP-950 has more bass impact. Both have extended taut low-distortion bass but I think the ESP-950 with it's larger diaphragms is capable of a little more excursion, so you get more bass 'punch.' Not as much as from good planar or dynamic headphones, but more 'punch' than other electrostatic phones.
So that's really the difference- the ESP-950 has a bit of 'up front, midrange-detail & bass punch' character while the SR-007 is smooth / mellow / neutral. I'd say the SR-007 is closer to a "reference" or "lab standard" sound, while the Koss has been given a slight "voice."
Both are VERY VERY good headphones, capable of very revealing audio presentation. Both have very very low distortion and very good time-domain behavior indicating very tightly controlled diaphragms with very little in the way of resonance or stored energy.
Also: While it is not made of luxury materials like the SR-007, the ESP-950 is more comfortable. All that Koss plastic is lighter than the aluminum used in the SR-007.
KOSS ESP-950 WATERFALL PLOT http://www.head-fi.org/t/566929/headphone-csd-waterfall-plots/30#post_7724844
One last thing: Koss ESP-950, LIFETIME warranty. SR-007, one year warranty.