David, I'm in something of the same boat as the OP, looking to upgrade and generally casting about in the $400ish space (although that's not necessarily a hard cap).
I had the chance to spend a few hours at home with a pair of HE-400s. I thought they were quite detailed, imaged beautifully, and had a realistic soundstage. I did notice a little weakness in the mids—a little "flat" sounding, there, versus "lively"— but the treble spike others have described may have suited my middle-aged ears quite well. Basically, I liked them quite a bit, but would appreciate your expert perspective before I make the jump.
So, imaging, space, detail, response...if those are my criteria, in something resembling priority order, are the 400s a good choice, or would you recommend something else in the same general price range, or a little higher?
Right now I'm using an Audio-GD NFB-12 as my DAC/amp. To complete the picture, if that matters.
Thanks in advance for any thoughts.