Comparing the Frequency vs Impedance graph reveals that the HD650 impedance in the bottom octaves is a bit more solid state amp friendly. Both the Asgard 2 and the Bryston BHA-1 are more than capable of driving either but the HD650 has a firmer grip on the bottom octaves which is confirmed by listening and sonic signature of both. What the HD600 leaves on the table in terms of the bottom octaves, it more than compensates with outstanding bass resolution. The HD650 obviously need more break in time to loosen up a bit.
Needless to say both the HD600 and HD650 with their rather tough impedance load are a challenge to drive properly with the typical iPhone, iPad or similar use. Not that you can't enjoy them that way but you will not get the full spectrum of dynamic contrast and sonic beauty.
Need a good SS amp like the Class A Asgard 2 or similar. Better yet, Tubes that can drive these 300 ohm headphones and like their higher impedance.
Higher impedance loads are hungry for voltage and the iPhone with it's 1.5v output is no match for the 10.9V Bryston BHA-1 capability at high gain.
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