I'm right on the edge of trying the D02 - the H40 sounds fantastic, but my budget is really very limited. The D02 sounds like it's got a unique overall sound, and is masterfully made/tuned at the same time, while being the same price as some recent W-shaped kings like the IT00 and the NF A2. I love and have been most impressed by single DD IEMs during my "travels" so far.
I am about 85% happy just sticking with my Blon03s, and they're certainly the best bang for buck I can imagine. Around $80 for potentially an all-round upgrade does not sound unreasonably though, especially if I went on to sell the Blon's to ship away at the expenditure even more.
It's hard for me to pin down what FR I like - I have loved the ER2XR, the SSR, and the Blon03s just about equally - and I know the Blons are the odd ones out on that list of 3. I do love the added bass and soundstage on the Blons, and I think the D02 would deliver there?