If you're wanting something really smooth and sustained (a la Scofield, Larry Carlton, Scott Henderson, etc...) try the Xotic BB Preamp. It can do a great clean boost all the way to hi gain sounds but is extremely quiet. With the gain about 12:00 you don't hear a ton of grit and it sustains forever and sounds really smooth.
That's just one of many options but one that I'm familiar with.