hd6xx is the same as hd650. the difference is just the paint job.

hd58x is very similar sounding to HD660s, but with slightly more bass quantity therefore slightly less neutral (emphasis on slightly), otherwise hd58x is probably 95% of hd660s.

hd660s has more clarity (slightly less mids, less midbass), more extended bass (subbass) and highs than the hd650.

to me, hd660s/hd58x is the modern take on the classic hd650 sound, but if u are a HUGE fan of HD650's flawless mids (can be a bit shouty at times, but still no other headphones can compete in this department), u might not like hd660s as much because of the change in sound. of course, the sound difference isnt really that big actually, but it is noticeable.

See my earlier posts:

https://forum.lowyat.net/index.php?showtopi...#entry103548679
https://forum.lowyat.net/index.php?showtopi...#entry103950808

So far so good, and it's not as expensive as the original pads (what a rip off!). Only problem is they don't come with adapters attached, so you'll have to somehow "glue" it to your old adapters. Mine keeps falling off every few months, but at least the skin cover hasn't disintegrated yet after more than two years of continuous use. Pretty comfortable too.