Sorry, I was busy at work all day... So I didn't get to reply to this until now.
Here's the issue I have with the HD280's. Yes, they are terrific "monitoring" headphones, which by definition means they are very revealing, but headphone at this price range always sacrifice one thing for another. In case of HD280's, they have a very good high through midrange presentation, definition and clarity... and they fall flat on their face when they extend below the midrange.
The trebles on the HD280 is not without fault, the sibilance gets pretty bad, also due to the nature of monitoring headphones, and the details are enhanced because there's a lack of overall fluidity and tonal quality. They are good for headphone in their price range, and if you're going for detail and definition, but they're by no means a complete headphone.
A500 isn't either, but it does better than the HD280 because it has a much more balanced sound signature. It has equal amount of treble and bass, there's no weighing towards one or the other. And unlike the V6 (another headphone in HD280's price range, but instead of lacking low-end, it has low, highs and mids and lack the sounds in between), the sound spectrum is filled out and full. Sound is nice and fluid.
Yes, it does have a few drawbacks, I think the strange effect with the vocals (it's still a very subtle effect) has something to do with that lack of frontal imaging, as most vocals are supposed to be projected in that direction. Something about the vocal gets a little flat for some weird reason.
Yet, I wouldn't say it's any worse than the HD280's... HD280 is a good entry-way in showing what sort of detail audiophile headphones can provide, but it's only part of the story, because it presents detail in detriment to the musicality itself.
