Well, the D1 is supposedly 100mW @ 32ohm and the E17 is 220mW @ 32ohm (E09k is 900mW @ 32ohm). The D1 and E17 do sound different since they are using different DAC chips (D1:AKM4396, E17:WM8740). I wouldn't say either sounded bad, just different. Keep in mind that I'm still kind of a newbie to this hobby so my impression is not super critical and full of audio buzzwords. :)
I prefer the D1 since it's USB powered and I'm not constantly charging/discharging the Li-ion battery in the E17 and burning through its finite charge cycles since my primary use is on the desktop just sitting there. That being said, the D1 alone is under-driving the HD598 a bit since I did notice an improvement after adding the E09k to the chain. I could hear/feel a bit more bass impact and the female vocals sounded better (Adele's Rolling in the Deep). It was a slightly noticeable improvement, but not night and day difference like it was with my AKG K702.65 since the D1 alone was massively underpowering my AKGs. Powering those from the E09k almost made me wonder if I just got a new pair of headphones they sounded so much better, mostly notably the bass impact/feel.
If you don't need the portability of the E17 I'd say get the D1 and maybe an amp later, like the Fiio E09k, Schiit Magni, Schiit Vali, or JDS O2 if you're on the budget. The only reason I got the E09k was that it was available at a local store, was on sale, and I could dock it with my E17. That and importing the Magni/Vali into Canada would almost be double the cost of the E09k. If I had to buy it all over again from scratch, I'd get the D1 + Schiit Asgard 2 since I want to get the Alpha Dogs in the near future. I think the Schiit Bifrost w/USB is a bit too pricey otherwise it would be Bifrost + Asgard 2.