An 8V-10V line driver with all gains in a system turned completely down should not go into clipping mode until ~90% volume, but distortion should have appeared by then anyway.
Rectify by getting a line-driver with a "Master Volume". Turn all volumes up on the system, then slowly start turning down the master volume on the line driver untill all distortion is gone. Then just use HU control for adjusting volume from then on and all will be fine.
