The HD598's impedance is 50 Ohms. Using the 1/8th rule for damping the output impedance of the should be less than 50/8 or less than 6.25 Ohms. I prefer 1/10th which would be 5 Ohms.
The phones output is too high and would cause a poorly controlled bass boost due to poor damping and the HD598's impedance curve. You could instead put the cans on the speaker taps, however, if you do this directly you can play far too loud for your ears and possibly burn out the 598's if you turn up the volume too high. The way to deal with this is to attach your cans to the speaker taps by way of an L-PAD which would lower the power delivered to the 598's as well keep the impedance to the cans low, like 5 Ohms or less and drop the power. You can find out about what an L-PAD is by asking Google. Basically it's two resistors per channel. You can also buy variable stereo L-PADs.
I am kind of worried now. I have been using my Grado SR-60i (32 Ohms) with my NAD amp. If it's output is too high for 50 Ohms HD 598, then I must be killing my SR-60s. The weird thing is, I don't hear anything unusual with the sound.....maybe it's there but I am just not listening for it?