Knocking down the high spots with a sander helps a ton, especially when you are talking a 1/4" max.
A layer of tar paper, or two, helps more.
Self leveler is a problem with nailed in floors, unless it is mixed with latex or acrylic, as it cracks when nailed and sounds "crunchy" under the floor.
Prep work is what you pay professionals for, the installation is simple.
