Gain and headroom are seperate issues in engineering terms. More or less gain does not impact headroom per se. The "gain" of a preamp is not adjustable anyway, it is fixed by it's designer; you are just regulating how much signal is fed to the outputs with the volume control. Like a garden hose; the water pressure is fixed (or should be!) and as you turn the spigot, you are just letting more or less water through. Same with a preamp, only less wet....