Thanks for everyone's input once again!
Haha, this just goes to show how much I still have to learn about headphones :) Thanks for all the corrections and recommendations, I really appreciate it!
I do value female vocals a lot since I listen to a lot of musicals, so perhaps the HE-400 are not for me after all.
I've read that my HP100's possess rather neutral sound with extended treble, and I enjoy them quite a bit for my music so far (though I wouldn't mind a bit more bass "impact", though I don't really understand yet whether I want bass extention in the "sub-bass" or "mid-bass"...). I think these do not have a V signature, so I would probably notice recessed mids if I changed to a headphone with a V signature... hard to say without auditioning more headphones though haha :)
Do you feel that the K712 have enough bass extension compared to the HP100? I've read that the K712 completely lacks V signature and have great mids, but then again I thought I read that the HE-400 had good mids as well.
Also, Silverwind helped me with some Omni questions in this thread as well as PM's earlier :)
The K712s mids are a tad recessed, they are good mids though. The bass extends quite deep and with the right pairing has plenty of body and presence. Some headphones with more midrange presence are the K612, HD 600, and HD 650. I think you will really appreciate the beauty, tonality, and depth of the vocals both female and male from the HD 600 and HD 650.