I currently own the XB500. I needed some cans with good bass without hurting my wallet. I got them online for $50 shipped and I can assure you, they're well worth it. LOTS of bass, but not overpowering. SUPER comfortable pads, but they are large and makes you stand out if you're wearing them in public (oh god, I can't imagine the XB700..). For $50, you can't complain.
I bought it out of my own judgment and curiosity. There aren't much reviews here at Head-Fi, so it's not very well known here. I didn't think it would wow me since it was $50 and it was Sony.. but after a few hours of constant burn it, it continues to wow me. I never tried to XB300 or XB700 but I do need some more portable cans so I might look to get the XB300 as my next cheap portable. Hey, for like $30, I can't complain right?