I've just recently gotten into Periphery from watching Nolly's video cover. They're awesome and Bulb is the sh!t!
Thats awesome :) Periphery rock very hard, and Casey is a fantastic vocalist.
Unfortunately there isn't too much of a precedent to follow, as far as heavy, technical bands with predominantly clean vocals is concerned, but that's a good thing in my eyes, certainly far better than copying other bands.
Protest the Hero have clean vocals for the most part, and it works amazingly. The singer has an incredible range, and one of the guitarists does occasional low screams, so their music pretty much runs the whole gamut as far as vocal styles go. Definitely check them out if you haven't already.
SikTh have some incredible clean singing parts, with some very very well thought out harmonies and rhythms. If there is one band
everyone needs to listen to on a regular basis, it's these guys. The brilliant songwriting, combined with incredible musicianship and an entirely unique sound makes them my all-time favourite band.
Textures are an amazing band that not many people seem to have heard about. The singer has a great clean voice, and also does the whole range of other metal vocal styles. He was pitch-perfect when I saw them live last weekend as well.