I recently upgraded from the Q701 to HE-400 and I'll try to summarize.
Bass -- So much better on the HE-400. It goes just as deep, if not deeper, hits much harder, and is faster. Even though it's faster, it's not necessarily "tighter," since the HE-400 has more midbass. I don't find the midbass bloated though. Overall, the bass has much more impact and more of a physical texture.
Midrange -- More recessed compared to the Q701, but it has a fast, tactile edge that really sounds nice on distorted guiters and synths. More crunch.
Highs -- Maybe not as extended as the Q701. The HE-400 are less bright overall, but there is a peak in the treble that gives it some sparkle (or fatigue, depending on how much you like highs). I thought the highs on the Q701 were smoother, but also more emphasized.
Soundstage/ Imaging -- The HE-400 have a smaller soundstage, but not by a large amount. I wouldn't call it small, just not huge like the Q701. However, the imaging is significantly better and more precise. I found the Q701s emphasized width but could also sound too diffuse. The HE-400 are more focused and 3D. You are closer to the music and the center image is denser.
Detail -- A bit more detail through the HE-400. The Q701, with their brighter sound, can make small details louder and more obvious, but the HE-400 reveals more in the end. You have to listen more closely though.
Overall, the HE-400 have more of a "v-shaped" signature, with a focused and precise soundstage that's moderately smaller. The imaging is much more defined. The HE-400 have a lot more midbass and a denser, thicker sound. Somewhat paradoxically, though, the HE-400 is also faster and more resolving. Sounds snap in and out of a nice, black background, and there is a tactile and precisely textured weight to the bass and midrange.
For classical, the Q701's soundstage has a clear advantage. It does a better job of recreating a grand sense of scale. However, the HE-400's imaging can be very pleasant. It's cool to have a clear picture of how different sections of a choir are positioned relative to each other, for example. The HE-400 does a better job of giving weight and texture to instruments like organ, drums, and horns, but the Q701 has a more natural timbre overall. The HE-400's treble isn't as smooth and it has a slightly off upper mids to treble transition. For vocals, the HE-400 are more recessed by I get a better sense of the physicality of the singer's mouth, how the lips move and the tongue interacts with the teeth.
Hope that helps.