^ OK, the diff between the S500 v. S400 v. ES700 v. WS99s to me.
JVC S500: They have a linear sound that is tilted toward a V type of sound but to me they are pretty balanced. None of the sonic spectrum sounds recessed but they no doubt have more emphasis on the lows and the highs rather than the mids. The bass is relatively fast, tight and goes pretty deep with a medium-large soundstage. A great set of cans that does most genres justice esp. most contemporary types of music like pop, EDM, and rap/ hip-hop, etc. A stupid good bargain for the price with this type of performance.
JVC S400: These are mid-centric cans no doubt about it. Their lows and highs are adequate but is nothing to write home about. Others have reported with lots of burn-in the highs will really open up, but for me they are still lacking the extension i get from the S500s. The star of the show on these is their lovely mids, which almost make the S500s feel almost a bit lacking. Due to the mid focused nature of the S400s, they are an easier/ relaxing listen and many have described as more "musical" when compared to the S500s. Overall i would say the S500s is the more "balanced" headphone out of the two.
ATH ES700: Like the JVC S400s these are mid-centric monsters. The mids on these are at least one tier above that of the S400s. Despite the mid-centric nature of these ES700s, the lows and highs are much more forward than the S400s. I would even say they are almost like a mix of the best qualities of the S500 and S400s all fused into one but with even more refinement added on top. With the stock pads the bass quantity is slightly less than the S500s, but quality is better, yet with the WS55 pads the bass quantity is on par if not more than the S500s. Therefore the ES700s is easy on the ears sonically and is relaxing to listen to, yet is also extremely engaging due to their refined bass and treble quantity and quality. The soundstage is also much bigger and more spacious on the ES700s compared to the JVCs.
ATH WS99: These are bass kings amongst all the others. Similar to the S500s i find these WS99 relatively "balanced" despite their overt emphasis on the low end. Nothing in the sonic spectrum feels lacking, the mids are forward and the highs are well extended along with their solid deep bass. The bass on these are so deep, tight, punchy, and relatively fast that it can be cup rattling without bleeding into the rest of the other frequencies. I also find the sound on these pretty linear, meaning that their presentation is more, for the lack of a better word "simple" and less intricate than the ES700s. Maybe it is due to the WS99s emphasis on the low end, the mids are slightly compromised when compared to the ES700s, though it's pretty close overall. The mids on the ES700s just wraps you up with its warmth and just sounds so magical. LoL The WS99s has a more aggressive bassy sound that is perfect for mainstream contemporary music just like the S500s, but will do other genres justice no doubt. I would say these are a complete and total upgrade to the S500s, esp. for those that love their bass. These are audiophile basshead cans 100% and is what i like to call Totalphile gears that does it ALL without having to compromise anything.
Hope this helps. : P