I don't know that veiled or dark are appropriate terms for the 598, as they do have some boosted treble to my ear. To me they sound a bit warm/dark in the low mids and somewhat bright in the mid-treble region. Overall, a relaxed, laid-back, and yes, a bit distant sound. Definitely not in your face or forward sounding.
Please define what treble "smoothness" means to you. To me, it means full and realistic, lacking edginess or grain. For many, it means a flat response or a rolled off treble. I don't know about boosting the treble between 1 and 10 kHz, as the 598 has been measured by multiple sources and shown to have peaks around 5 and 8 kHz.
Regarding differences in sound and frequency response of DAC/Amps, I'm no expert, I just know that they can have a very noticeable difference in the tonal balance (frequency response) of the headphones once mated to them (not testing FR of DACs by themselves). Maybe something to do with varying impedance at different frequencies. I don't know. I just know what I can hear. And from the few portable DAC/Amps I tried, there were very noticeable differences in sound. Audioengine D3 sounds warm and bassy. Audioquest Dragonfly v 1.2 sounds neutral with a touch of brightness. Ibasso D-Zero MKII sounds quite bright with its built-in DAC, but fairly neutral when used with my iphone 6. Meridian Explorer sounds fairly neutral, maybe just slightly warm. Fiio e10k sounds fairly neutral. This is when listening with the 598s, which of course, are not a reference can with dead-flat frequency response, but relative comparisons can still be made.
For me, the best synergy with the 598 come with the AQ Dragonfly 1.2 and the Fiio e10k with bass boost on. The DF 1.2 can sound a bit bright on some songs (although the treble is very smooth), and is a bit lacking in the bass punch, but has excellent detail resolution and a very open, spacious sound. The e10K lacks the DF's smoothness, resolution, detail, spaciousness, and somewhat more refined sound, but it's a bit more fun to listen to with the bass boost while also easing up on slightly bright treble that can plague the DF on some songs.
Thanks for link to custom EQ settings. I use EqualizerAPO on my laptop, so I'll check them out when I get a chance.