The V600 is pretty similar to the HD500 that I used to have (yup, pre head-fi). Soundwise: lots of midbass, grainy mids, no deep bass or treble extension. Very poor detail, imaging, soundstage, etc - as a matter of fact, you wouldn't even know what imaging and soundstage were just from listening to them. They were meant for kiddies who want big bass with their commercial hip-hop and wouldn't care about anything else, or know any better. To top that off, these were bargain headphones in Germany, but in the US, they commanded a hefty price tag for what they delivered.
As far as what you want to get: The A500 wins out over the D66 sound-quality wise for what you intend to listen to. But, that was a given. These are two very different sets of cans - the D66 were designed for portable use, and are small and foldable, while the A500's were made for stationary use, and are absolutely enormous (though very light). The D66 has a very natural, even sound, but is low on bass - or so I heard. I haven't actually listened to a pair, but that is what people seem to think of them. The A500's I owned, heard multiple pairs, given as gifts, and always thought very highly of (now upgraded to A900's, which are also really good). The A500's have a nice, neutral sound, with pretty big bass though well controlled, clear, natural sounding mids though a bit recessed, and a crisp treble, though not harsh. Are they fatiguing? Out of the box, they probably will be, though nowhere near the SR80. Once they break in, the sound smoothes out a whole lot - the treble settles down, the bass comes out and becomes more controlled, the mids become a bit more upfront. Also, they have a pretty nice soundstage. Very wide, almost unnaturally so, and imaging isn't the most precise, but hey, this is a $100 headphone - most headphones in this price range haven't even heard of soundstage. They will be really comfy, and are pretty well built, though the cord isn't detacheable. Overall, a very good bet - if you don't care about portability. If you do, D66 might be a better idea.