Well, in my opinion (Pre)-Vanishing Vision is speed metal, Blue Blood is speed metal with symphonic elements (some songs are more like hard rock, though), Jealousy is a weird mix of hard rock (Miscast, Joker, Desperate Angel), power metal (Silent Jealousy), speed/thrash (Stab Me In The Back) and ballads (Voiceless Screaming, Say Anything), Art of Life is symphonic/progressive metal and Dahlia is alternative/progressive metal.
In general X should rather be categorized as speed metal or symphonic metal than power metal, though, since they have only one or two power metal songs.