I’ve been in the process of re ripping my whole music collection in lossless and these headphones play everything clearly and with great detail

Bottom line, if you can afford the HD660 and appreciate extra detail in sound, a wider sound stage, will use for music and gaming, go for it

The detail is simply stunning, 2.5mm balanced cable (purchased separate cable) or the supplied 3.5mm cable.

Soundstage outstanding, every instrument in the orchestra stands out in exciting detail

I won't dare call the GSP500s muddy, but they seem to have a bias that is a little deeper, and lose some details in the mids

The bass will not blow your socks off but the detail and clarity is outstanding and for an open-back headphone it is more than sufficient for my needs.