I just got my Koss ESP-950 in the mail and I figured I'd throw in my impressions as compared to my STAX SR-5/SRM-1Mk2 Pro setup. My ultimate plan is to construct a Koss>STAX adapter so I can drive these ESP-950s with my SRM-1Mk2 Pro, but for now I am just listening with the included E/90 amp. I don't really have any experience reviewing headphones, so bear with me as I don't know much fancy audio jargon.
I have been listening to the 950s and the SR-5s side by side for a couple of hours now, and I have covered most of the genres that I typically listen to. I will offer my opinions based on genre below.
I started off with some prog rock in the form of Yes's "Fragile", "Relayer", and "Time and a Word" albums. Listening to these albums on the 950s, I couldn't help but feel they were missing something compared to the SR-5s. Much of the detail I was used to hearing seemed muffled and quiet. Unlike the SR-5, the bass impact is actually present on the 950, which is nice for a lot of music, but in this case seems to be the source of the problem. I don't think that the 950s resolve poorly, but it's as if the greater bass presence kind of overpowers a lot of the subtle details in the treble. When A/Bing the 950s and SR-5s I found that I could listen to the SR-5s at much higher volume levels than the 950s without discomfort due to the SR-5's lesser bass impact. I believe that the higher listening volume of the SR-5s is responsible for bringing out the finer details in really dynamic tracks such as the ones I tested. I did not listen to any classical music on these yet, but I suspect that it would suffer from the same issues as my progressive rock collection. By no means am I saying that the 950s had a poor showing here. Quite the contrary, I found them to perform phenomenally when compared to my dynamic cans.
After the progressive rock, I listen to a few Primus tracks. I expected the ESP-950s to perform exceptionally well against the SR-5s with Primus due the bass driven nature of their music. In general they did perform very well. Songs like "Mr. Krinkle" and "My Name is Mud" were represented exceptionally well, with satisfying and impactful bass which just was not present with the SR-5s. Strangely I do feel that while the bass slam is much better on the 950s, there is something about the texture of the bass that the SR-5s deliver that is a bit more convincing. I'm just nit picking here really. The bass slam that the 950s provided made the experience of listening much more visceral, and that is exactly what I want from this kind of music.
Metal is really the ESP-950's wheelhouse. I started off with Moonspell's "Night Eternal" album and whoa! What a rush! These 950s just dominate! The experience is visceral and powerful, yet still detailed. The SR-5s present a rather dull but detailed representation of this genre, kind of analytical. The 950s are warm and satisfying, which I credit to the harder hitting bass and the forward midrange. I followed this with some of Nightwish's "Angels Fall First" and "Dark Passion Play." Again, the 950's just performed. Iron Maiden's "Powerslave" sounded great, System of a Down's "Hypnotize" was excellent. I even listen to some recent Rush albums "Vapor Trails" and "Snakes and Arrows" both of which sounded phenomenal. I don't really know how to explain it, these are just the best cans for Metal and Hard Rock music that I have heard. I have no complaints. The detail and speed is 100% electrostatic, but the bass impact and forward mids sound absolutely like dynamic phones. Also they are much more forgiving to low quality recordings and low bitrate MP3s than the SR-5s, which is especially important for these genres.
Well that's all I got for now. Overall I'm quite glad that I bought the 950s. At this point I still think that I like my SR-5s better for Progressive Rock, which is my favorite genre, but for most everything else I listen to, the 950s surpass them. I hope that switching the 950s over to my SRM-1 Mk2 Pro will fix some of the problems I have with the them, but even as they sit right now I am quite pleased with them. Also I wanted to point out that the build quality is fine. It's all plastic, but it is sturdy plastic that doesn't feel like it would break easily. You might expect more from a ~$750-900 set of cans, but these are electrostatics with an included amplifier, $750 in electrostatic money =/= $750 in dynamic money.