I think the HD-650 is a terrible portable. It is large, hard to drive, and easily dented. The D7000 is a bit big, but far easier to drive. The HD-650 has problems with being too laid back ( sounds like the audio is going through a pillow ) and has a frequency dependant impedance curve. Even through my m903 the HD-650 did nothing for me. Heck, the HD-800 was just as lifeless to my ears. I had to turn the volume up on both to get the sound I wanted and at that point my ears would start to hurt. I vote neither.
I am with dadong here. The DT1350 and Senn HD-25 ii would be much better because they are way more portable. They are also much easier to drive due to the lack of frequency dependence in the impedance curve. The D7000's are not going to be any better than the D2000 and many of us prefer the sound out of the D2000. For shere sound quality on portables I go IEM every time. I can show you a dozen IEM graphs that are nearly perfect in frequency response, distortion, 50Hz and 500Hz square wave response, as well as isolation. The cost of such IEM's? Certainly less than the cost of the D7000's. For example the UE700's are amazing sounding for the $150 price range. Other than some warranteed build issues they are a really nice pair of IEM's. Even the Brainwavz B2 is going to be less expensive and would sound just as good without a need of an amp.
If you are dead set on headphones and you do not mind the bulk then the AH-D2000 or DT990 ( 32 Ohm ) are great headphones that scale extremely well with head amps. I own both and enjoy each for what they are. The D7000 is an overpriced and problematic D2000. At this price point you might as well save $50 and go LCD-2 Rev 2 which is going to out perform both the D7000 and HD-650 and sound a ton better. The LCD-2 is made in the USA too! The LCD-2 does not have any wacky frequency dependence in the impedance curve either and has a wonderful sound reproduction be it a tad dark at times.
Pick the D2000 if you want a closed can that has deep bass and sounds amazing, but lacks a little on the high end without a good amp
Pick the DT990 ( 32 Ohm ) if you want an open can with really quick bass, a great mid range, and a decent amount of treble.
Pick the Q701 if you want the HD-650 sound sans the "veil." They are super comfortable, have an awesome mid range, and make percussion sound amazing.
As far as the iBasso D6, which I have not personally listened to, I know many people really like that amp. The DAC is the WM8740s which is a nice DAC chip. The amp should have no problem driving the DT990, D2000, but may struggle a bit with the HD-650.
Edited by NA Blur - 10/10/11 at 3:02pm