Here's my view since I was originally going to purchase the XB500, but ended up going with these HAS500s. Now, I also ordered the XB500 and it will be here tomorrow.
When I first opened the HAS500s and listened to them, I wasn't blown away. I thought I bought just an average pair of headphones that all these people were hyping up. I put the Sennheiser Adidas blue pads on them so they look cool but I wasn't "wow'd" by the sound at all. However, after burning these in for over 100 hours now, I can tell you that they sound amazing. I was surprised to hear things in music that I hadn't noticed before. I'm blown away at how amazing the new Daft Punk album sounds with these. You can just tell that you're hearing music the way it was supposed to be heard, even though these aren't the most neutral phones you can get. For 90% of the music I listen to, I'm going to use these. I'm actually picking up another pair in gunmetal soon.
The reason I'm also getting the XB500 is because I also listen to some really bass heavy EDM... electro-house, glitch hop, and even some rap. Sure, I could probably get much better bass canons for $200+ CAD, but I found these at a good price. For this last 10% of music I like, I want bass that rattles my ears. Since I heard that the XB500 EQs nicely, and since the bass is already in them, and considering the price, buying these is a no-brainer for me. I also rockbox'd my sansa clip+ so I have great EQ options to work with.
tldr - For all quality, live instrument music that I want to listen to, I will use my HA-S500s. For synthesizer/bass heavy music, like some EDM stuff, where I want my ears to rattle, I will use my XB500s.
The s500 sound better with the blue paddle that you put it?