I don't have the 400, but I am quite familiar with the Q701 (I auditioned it for quite a while). If cost doesn't matter too much to you, I would say get the 400i. The mids and highs on the 400i are even more detailed and natural sounding than the Q701, but you'll also get a lot more needed bass with the 400i. Also, once you do the grill mod (or just take the grills off of the driver altogether) I find that the soundstage size is pretty much as big as that of the Q701's.
The only genre of music I don't really listen to out of all that you listed above is electronic, but I'll give my impressions on how the 400i does with the other ones. 400i vocals are excellent - really detailed, natural sounding, and they are intimate but not too "in-your-face." And no sibilance and fatigue with female vocals (unless the track is mixed really really badly) as far as I can tell. While the Q701 also did well with vocals, I found them to be too close to me and more easily fatiguing than the 400i's vocals.
I believe the 400i does pop better than the Q701. Mainly because the 400i has that needed bass and that its fairly forgiving of somewhat badly recorded material due to its somewhat smooth highs and mids. The Q701, because of its mid and high focus and the "sharpness" of those frequencies, to me is less forgiving of less than optimally recorded music.
400i hands down does jazz better than the Q701. You need some of that bass to make jazz sound good, and thus to me the Q701 is missing a good chunk of what really makes the jazz experience. Plus the 400i's bass quality is so good, and that also really adds to the music.
Classical - without the grill mod it's debatable whether the Q701 or 400i is better. Q701 sounds highly detailed in mids and highs, and bass doesn't matter as much in classical, so it does well here. However, the 400i is at least just as detailed at the Q701 in classical (I personally feel its better, but others might disagree with me). The Q701 sounds more airy than the 400i without the grill mod, which is good for classical. But with the grill mod, I find that the 400i doesn't really lack in air compared to the Q701. Plus, I feel that imaging in the 400i is significantly better than the Q701 - this is great for orchestras.Overall, for me the stock 400i slightly edges out the Q701 in classical, but with the grill mod the 400i is definitely better than the Q701 for this genre.
Great comparison and contrast between the HE-400I and Q701. Your keen views on the HE-400i are spot on.