I also notice the MDR-XB900 are available now - I wonder how they compare but they are also quite pricey at $200 (incl postage) into Australia from Hong Kong where the XB700 only cost me $70 (from the same country)
I have MDR-X10 (almost identical to XB900, only color variant). Its sound is extremely over bassed and unclear. On-ear pad is not comfortable compared to XB1000 (I do not have XB700). I dislike X10, so I will not recommend X900 to anyone.
Here's how I'd describe the difference between these two using a simplied frequency response graph that would picture how I see the relevance of the lows and mids and highs to each other in these headphones based on how I thought they sounded unEQ'd and how I EQ'd myself and comparing a bit with headphone.com graphs:
XB700 has a quite typical V-shaped frequency response curve while XB500 is more like a slanting slope.
From my experience, the Sony XB500 has more midbass than the Sony XB700 headphones, which is why it has more impact and feel. If you prefer deep subwoofer bass over midbass, then get the XB700 headphones. If you are after headphones that have a great bass impact that you can feel XB500 all the way!
Your frequency response is wrong. The biggest difference is that the xb700s are boosted around 2khz by about 3 to 5 decibels louder than the xb500s. And also the xb700s reaches their peak in bass around 200hz and slightly starts fading from there.
Having owned both headphones, I can say that the Sony MDR-XB700's are more comfortable wear, and definitely offer more in the low frequency range. When connected to an amp, the 700's physically vibrate
. Great for bassheads. Now to acquire the XB1000's ...