My HD650 sounds good from the headphone out of my SCD-XA5400ES SACD player. Sure, its midrange does't bloom and there is a high-feedback kind of grain to it. But there is also a very literal, flat, articulate rendition of information. The frequency range is extended and the headphones sound a bit more neutral. This is compared to my Woo Audio WA6 with rolled rectifier. There, it loses a bit of emphasis on detail and instead offers a smoother and more realistic texture. One forgets that it is a stereo a little easier and lets go into the music.
My M-Audio Firewire 410 does a credible job, better than the headphone out.
So, no, it doesn't need a dedicated amp. It is easy to drive. My iPhone can't get enough volume, but otherwise the HD650 makes the 'phone sound as good as it has.