To throw a bit of a wrench into this conversation ... Sometimes, there's literally no difference at all. When I worked for St. Louis Music, we built an updated version of the Ampeg V4B head (late '90s, early 2000s, can't remember exactly when). It was literally a modified version of the Ampeg SVT-CL tube preamp with a modified version of the Crate Blue Voodoo 100w tube power amp section. So, in a sense, it was kind of both.