This comparison is difficult for me as I might be overlooking an obvious explanation to what I hear. Tonality and frequency response is basically similar, but RE400 has a stronger high roll-off and has more focus on mids. Both are fairly neutral with a slight emphasis on mids.
Yet the presentation I perceive is completely different.
Extension in the lows is similar, but the sub-bass is perceived stronger with RE400 although it sounds very lightweight. The bass of the SD2 sounds much darker and realistic, although punch is a bit softer. This makes the HiFiMAN sound a bit cleaner, if artificially so.
The RE400 focuses strongly on mids, presenting them in a relaxed way. The distance to voices is bigger and thus SD2 can deliver more intimacy. Maybe due to a mild lower mid boost, voices have much more volume and body, sounding more realistic to my ears. They are not as soft as with the RE400 but never fatiguing; probably because the upper mids take a dive.
Highs are recessed with the RE400 whereas they are linear to slightly recessed (depending on tips) with the SD2. Both are very clean with even transitions and a careful peak at 10 kHz without any hints of sibilance. At lower volume the SD2 extends further. Even though the performance is very smooth, RE400 has stronger roller coaster rides making the SD2 the obviously better choice for monitoring.
Width of soundstage is comparable in size but the RE400 lacks quite a bit of depth when compared to SD2. I’d say it is similar to W4 that extends nicely from left to right, but because of the bigger distance the depth appears a bit flat. Layering is better with SD2 and I really appreciate the intimacy voices can give without being too close like with SE535. Separation is good with both but I actually might have to give RE400 an advantage for resolution. The music is tighter with very short decay, putting it up there to triple (and some quadruple) BA universals. The SD2 is slightly behind.
The RE400 is very cohesive and everything fits well together. However, bass and highs appear veiled to me and they desaturate the image. Thus, timbre is off and the overall sound reminds me of hollow wood or paper.
The great timbre and convincing soundstage is what makes the SD2 stand out. Technically, the bang-for-buck of the RE400 is crazy and the presentation definitely does not cause any sort of fatigue. But it is also worth mentioning that I have little faith in the durability of the non-replaceable cable, that no carrying pouch is included and that none of the included ear tips fitted my medium sized ear canal. The build quality of SD2 is reference quality.