Potterpastor is correct, mostly; The difference between an Asgard or Valhalla (or any DAC/Amp) vs your Yamaha in terms of power is so subtle. Your Yamaha can easily drive the HD 600 or beyerdynamic DT 770 250Ohm with power to spare. I personally use an outboard DAC/Amp because I mostly listen to music at my computer and my Yamaha is sitting 15' away, but in terms of overall transparency there is no discernible difference*, only convenience. I also suggest ditching the DS2S if that is your DAC and get an Audioengine D1/D3 or a Dragonfly ; Both are inexpensive and can also be used as an amp to drive the HD 598. I actually used my D1 as the DAC and fed it out to my Yamaha.
Let me ask you this: Are you happy with the sound out of your Yamaha? Is there any inconveniences in the listening environment? Unless you have money to spend and want to start down the never ending path of Head-fi, stick with what you have.
Tip: Your Yamaha has Direct and Straight modes. I suggest using Direct as it bypasses the tone control.
* There is differences in sound signature, but that is due to the components used. In no way is there any degradation of sound quality, added distortion, or lack of transparency.