[Stoney]

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. 

 

[skeptic]

Quote:

 
...no, it doesn't need a dedicated amp.  

Agreed ^.  I did a little listening to my 650's out of the HP jack on my Marantz SR6003 last night (which I typically use for movies with my DT880's), and I was pleasantly surprised at the quality of the output (after disabling Dolby HP).  In pure direct mode, I think this unit definitely deserves a spot on the list of receivers with decent headphone circuits.
 
The sound stage and separation may not have been quite what I get out of my WA3, but it was nonetheless very enjoyable listening.  Come to think of it, PRAT may well have been better on the Marantz.