Well the D1 uses the AK4396 DAC chip which is from Asahi KASEI. This chip is pretty good, but some claim that it has limited sound staging. According to the specs it should work fine and I recommend you focus on acquiring a great amp for the HD-650. The HD-650 is trick because of two main factors. The first is simply that it sounds veiled. The audio sounds like it is playing through a thick pillow before it reaches your eardrums. For me this was intolerable no matter how hard I tried to play it down. No live music sounds this way and a headphone should not deliver a veiled sound. That said if you can stand it then there is the second issue. The second issue is the frequency dependent impedance curve. See below:
As the headphone reproduces specific frequencies the impedance is changing. This makes it hard for an amp to control the voltage and power needs of the driver. Because of Ohm's law we know that V=I*R or Voltage = Current ( I ) * Impedance ( R ). As the impedance increases the current decreasing making it a tad easier for the amp to deliver the necessary voltage to the headphone. Sounds like a good thing right? Looking at Joule's law we see that P = I^2*R or P * V^2/R. Because most amps are constant voltage sources ( set by the volume knob ) we use P * V^2/R or Power = Voltage ( V ) Squared over Impedance ( R ). Now that you have higher impedance you can easily see that the power drops significantly. The harder it will be to power the driver and voice coils. These two laws make impedance a tricky beast. Back in the day they used high impedance headphones so you could connect many to the same circuit and drive them all with the same voltage and not have to worry about current, but they were rarely concerned with fidelity.
Here are three things to make the HD-650 sound better, but to be honest the HD-650 is one of those headphones that you will end up hating or loving. Time will tell.
1.) Remove the foam insert within each earcup. This easily pulls out and if you are careful you can easily place it back in. This will help unfilter the driver giving the headphone less of a veil.
2.) Find an amp that can drive at least 700mW into the headphone out with a voltage of at least 8 volts. Tube amps can typically deliver more current for less money so perhaps start there. Testing your HD-650 with a few amps is going to be your best hope in finding one that works well. For me the HD-650 still sounded dry, veiled, and in order to get decent sound too loud, out of my Grace m903.
3.) Going truly balanced may deliver bass and treble extension for the HD-650 making it more tolerable in the long run.
I say keep the D1 DAC and test a few amps and purchase the one you like the most. If you end up not liking the HD-650 go with a pair of Denon AH-D2000. They may be just what you end up liking in the end.
Edited by NA Blur - 7/18/12 at 3:36pm