This guy from xotic effects is what I use. Great clean-with-character-boost I use at the very end of my chain for about 2 db boost. It makes everything "go to 11" when I need just a little bit more. Now it is not completely clean, but I like the character it adds.
EP Booster
For completely clean/no color at all, the Zvex boost pedal, the Super Hardon, was impressive but it is really pricey. It had absolutely no color and no bass loss at all, just the same exact sound only louder.