I've had my Stax O2 rig for enough time now with a Stax SR 323s amp. I'm just going to post impressions because there are plenty of reviews that are more detailed and thorough than I can consider. Initial first day -Before the initial first day of receiving the 323s amp, I did not use any headphones for several weeks and just listened to music with speakers. -Slightly underwhelmed by the lack of organic attack. Vocals just don't sound as organic, intimate, and involving as the W11R. It's drivers are incredibly speedy sounding, but light. They feel like they push air faster than dynamics, while dynamics feel like they push more air but slower. Electrostats do not sound like they have drivers. They are so fast that it seems sound just appears. -They are the most non-fatiguing headphones I've ever heard, yet bass pounds hard. Not basshead hard, but definitely a more adequate amount of bass than I've gotten out of most top tier headphones. If you're worried about the O2s being bass-light, that is not a concern. What you should be concerned more about is the less organic sound. Bass impact is one of the O2's strongest aspect. It pounds and rumbles. -My ATH W11R has a very similar sound balance as the Stax O2 mk1. I'd even say the LCD 2, O2 mk1, and W11R all sound pretty similar but the O2 mk1 and W11R are a tad brighter. Vocals on the W11R just sounded more realistic and organic. - There is hum on very quiet orchestral pieces that require me to boost the amp's volume. The hum is from raising the volume on the amp. After half a month -The Audio Technica W11R have had almost zero head time. - The O2s have better layering capability than the W11Rs. Multiple and convoluted vocals, talking in the same volume and pitch do not blur into each other like they did in the W11R. A/Bing the W11R and the O2 mk1, I'm hearing lyrics of back-up vocals I couldn't hear from the W11R. -The fatigue-free O2s are just so much easier to listen to and I have become accustomed to their light and elegant sound. - A lack of organic feel of the music is not a concern anymore. It feels like the O2 reproduces music as it is, not adding any character of its own. Generally, I have felt most headphones added a character of their own. - Since the O2s sound so transparent, fatigue-free, bass is so aplenty, and the sound is warmly musical, I have had absolutely zero interest in changing headphones or trying other headphones. I'm now pursuing more music instead. - I've finally started changing my music in FLAC. I never felt it was worth the inconvenience until now. My EQ for the Stax O2 mk1 for my personal preference. I do find the cymbals to be a bit fake, sparkly sound without EQ. I also find it more pleasant sounding this way. I'm a poor guy, but I feel this is one of the best combinations someone can get in Head-fi. If there is a difference between an KGSS and the SR323s, I'll be sure to note it when I can. If anything, a KGSS will probably have the power so that hum wouldn't be audible on really high volumes. I can't really find much wrong with this combo. It just does so much right for the money and is the second best headphone rig I've ever heard. (The best I've heard was n3rdling's Blue Hawaii and O2 mk1 combo) My memory of n3rdling's rig is extremely vague, but I'm hearing all the major characteristics of the O2mk1 with the SR323s that I remember hearing on the BH which includes strong bass, rich mids, highly transparent, elegant and non-fatiguing sound. It might take a while to get used to such a transparent presentation of sound, but if you can appreciate it, then you can probably say "The End" to your headphone journey. I have and I will not purchase any more headphone related equipment. . . . . OK, maybe an Ultrasone ED8 when it becomes cheap because it's so dam sexy and portable.