difference between K400 and K501 (based on memory):
K400 has more treble, slightly more aggressive sound. Not as balanced (possibly), but pretty close in sound. K400 was a bit more fun than the K501.
The K501 seemed to have a much larger soundstage. K400 seemed to have less bass than the K501. You can use K601 pads for more bass though.
For vocals I'd pick the K400. For classical the K501. K501 seemed more balanced.
The K601 has much more bass than the K400, is much warmer and has less treble. I think the K601 might have a peak in the lower treble that the K400 doesn't have.
I had the K400 and K501 and still have the K601 and Q701. I sold my K501 when I got my Q701 since I found it better.
BTW the K400/K501 should require the same amounts of power. They're harder to drive than the Q701 and K601.
The ODAC+O2 should be OK I would imagine.
HD-650 and K400 are nothing alike. The HD-650 makes the K400/501 seem bass light in comparison.