Interesting subject! One I I don't have a firm opinion on.
Let me begin by saying that the D2 drives the SE530s very well on it's own. Well enough that for most music and for portable applications it makes you question the need for an amp. A very enjoyable listen without anything extra.
That said, the Tomahawk certainly adds warmth and depth to the music. This is desirable with the D2. The changes are quite noticeable. Clarity, depth, warmth, soundstage all improved.
But the question for me comes down to whether it is worth it for portable use, on the move. Most often for me the answer is no. I have a home amp so don't need a portable amp there. The gains are questionable when working out or doing lawn work, so the portable amp is left behind. In my case I find that I seldom use the portable amp with the D2/SE530 combo which sounds very well without it. In fact, this combo has wonderful synergy.
I have not heard other portable sources. My guess is the SE530s will magnify any weakness in your source. Many portable sources will probably benefit from the use of a portable amp in a big way ... much more than the D2. I also think usage dictates need. Just as a top car audio system makes virtually no sense in a noisy or loud vehicle, so too a portable amp may not make sense for active use or loud environments. Sure, the SE530s isolate as well or better than most IEMs. But when I'm riding my lawn tractor the environmental noise is loud enough that I won't hear the wonderful sound quality my portable amp adds to the set up.
At least that's my current thinking on portable amp use with the SE530s.