Also, has anyone tried the Audio Dragonfly thumb drive amp/DAC with the HD 598's? I've heard they produce tremendous sound for such a little thing, and on Amazon they're currently priced for around $100! A steal to be sure, but I would like to know how they perform with these.
I use the Dragonfly with the HD 598 and my laptop and desktop. It sounds very good. (Today, I listened to Miles Davis's "Kind of Blue" in full with the HD 598 through the Dragonfly, and it sounded better than ever.) I would say that, of all my headphones, the HD 598 is the one that improves the most when paired to the Dragonfly with respect to being used off the laptop and desktop headphone socket directly, with the possible exception of the TDK BA200 IEMs, for which the big improvement is that the Dragonfly is much quieter. I am still trying to compare the Dragonfly to using my NAD 3130 socket. I feel that the bass is more controlled (less bloated) with the Dragonfly, but maybe I am just being swayed by the recent discussion in this thread about the effect of high impedance phone sockets! Note that the Dragonfly has very low output impedance (see http://www.stereophile.com/content/audioquest-dragonfly-usb-da-converter-measurements).
On the matter of output impedance, NAD service confirmed that the NAD 3130 phone socket impedance is 220 Ohm (just as for the NAD C320BEE). Also, a post in the thread at http://www.head-fi.org/t/624407/nad-3130-headphone-section states that the headphone circuit for the NAD 3130 is simply a 220 Ohm resistor in series.