Amen, it's only possible to degrade SQ. I tried whenever possible to use a passive device when I could arrange source and amps with appropriate impedances and levels. My first was a DIY box with Alps pot and quality switches, then later a McCormack Line Drive was the sweetheart controller of my 2 ch rig for 20 some years. A jewel.All things being equal, ie appropriate voltage drive levels and impedances a pre amp can do nothing to improve the sound.