so basically you want more instrumental type songs then

I would look at rozen maidens (fair bit of piano and violin/chello), venus versus virus (again a few are piano based on there), polyphonica (mainly instrumental type songs), air (tv) (some of this may work), eureka seven (niji aka rainbow by denki groove is a personal favourite of mine), simoun (has a french feel to it in my opinion), appleseed/vexille movies as you've mentioned advent children, probably consider looking at ergo proxy too.
Lets see what songs I would look at (quick check on itunes)
rozen maiden (well most of the album is probably suitable

) -
Akumu by Mitsumune Shinkicki
bara no jibai by Mitsumune Shinkichi
Kodoku na Kokoro by Mitsumune Shinkichi
bara no chikai by Mitsumune Shinkichi
eureka seven - niji (aka rainbow) by denki groove
(not mentioned) burst angel - Shinjitsu no Kao by nishida masara
Air (ornithopter album) - Aozora by Shinji Orito - Magome Togoshi - Key Sound Label (beautiful song in my opinion)