[ext23]

  Not a lot of difference but the input specs of the Magni2, which the Modi is often used with, might make 1.5v a lot less of a problem than it looks.
 
Again though what may "seem underpowered" to you could only be partly a problem with sensitivity, power, and gain, and partially if not mostly because you expect a totally different sound. Instead of the flat 10hz to 1000hz of the HE400i, what you need is one with boosts from the upper bass to the midrange, like the Grado RS1.

 
Yes, since my first post I switched the LO on the X3ii to fixed and have been listening to the 400i on mid-gain at about 12 o'clock, which seems a lot more sane. I think I was trying to play it loud enough to rattle my head and now I'm getting used to the sound. Just trying to understand more of the technical side of things at this point.