Re unitary gain -- ideally you should operate your pre amp as close to unitary gain (1:1) as possible i.e. avoiding active amplification and avoiding excessive use of the volume control. To the extent you can manage gain upstream of the amp you'll find it better if you do this then if you send in too small a signal (and have to up the volume into the 6-12dB of available gain) or if you have to run the volume control at <30 on the 110 point setting. In my experience the range 40-60 (or 70) sounds best.
ps this will also obviate any risk in excessive gain in switching between sources, or inadvertently activating the pass through
ps this will also obviate any risk in excessive gain in switching between sources, or inadvertently activating the pass through