[skamp]

17.5mW into 300ohm

Then I'd say the D7 is inadequate for the HD 650. You'd probably get close to maxing it out.

 

[ProtegeManiac]

Quote:

I too had asked iBasso about the output power and the answer received was: 17.5mW into 300ohm
 
Or you can just add an dedicated amp, like the P4 to the D7 later on for higher impedance headphones.

 
Still likely to be too low, but there's still a chance you'll be satisfied for a while; then again, people either want things (too) loud and sometimes mistake plain loudness for better outright, or in my case, I went into (good) headphone (systems) coming from speaker systems with high-current amps. So why take the chance on just the D7? Your choices should then be either:
 
1) get headphones (Grado) or IEMs that would be great for the D7, and enjoy it for a long time; or
2) get the HD650, but be prepared financially and psychologically to spend more on an amp later on, given the higher likelihood that the D7 won't be enough for the HD650 (or not enough for the HD650 for your ears/preferences)
 
And in case you take Option 2, the P4 won't be on top of my list. It's still a portable, best go with a desktop amp with significant output advantages over portables, and preferably RCA inputs at the back (the P4's layout is designed primarily to have an iPod strapped on top) since the D7 has them too on top of the lineout 3.5mm out front. You don't have to get the new amp right away, I doubt the D7 would be absolutely useless with the HD650 - even portables at max volume with no more than 30mW@32ohms are loud enough, I'd assume the  D7 will be as loud and much, much cleaner, just not near enough to really squeeze more out of the HD650 - and use that time to enjoy them while you shop around for an amp.