You're most welcome, ...now get your GF a pair or else you'll not get any candy from her !
Haha funny thing I was doing a quick comparison with my brothers HM3's just a couple hours back.
The HM3 is more bright; less bass hitting, and treble and highs are a tad bit higher, can be piercing if you have sensitive ears.
These HA-S400's on the other hand are warmer, and have a good amount of bass with stock pads, no mods. The mids are just a tad bit receded. HA-S400's have less clarity and a smaller sound stage than the HM3's from what I remember.
I like to think of it as HA-S400 being more "new gen" with sleeker design, better portability and a bit more bass with the sacrifice of some clarity which the HM3's have.
I've a strong feeling your S400 has less than 200hrs of burn on them. the mids on mine are right up there with the bass and yet not diffused.