So leave the media player volume at 100% at all times and adjust volume only with the preamp. That's what everyone I know does.
(Not sure what the mixer would be for but in a high end system they are a rarity because of the extra circuitry in the signal path. I would try to bypass the mixer for music listening)
IMO having volume or gain settings set at 100% introduces output/input distortions on equipment. Best to back it off to a safe level. Also years ago when I first got the BDA-1 I compared it to the BCD-1 and I found that volume/gain set at approx. 80% I could not discern a difference between 'em so I sold the BCD-1. The level of computer output etc. effects how a dac is gonna sound.
The mixer was a brilliant idea I had for editing recordings. Not knowing how to use a mixer I played with the settings for two weeks trying to find where it sounded optimal. What I discovered was there is a fine line between sounding good vs sounding musical with the settings. So being able to customize the sound, I just left it in the chain.
The Yamaha MG-10 mixer distortion spec if IIRC is something like .01 so it doesn't mask the sound of my Brsyton's from what I can hear. Whereas using an equalizer in the past masks the sound. I tried finding distortion specs on other mixers and can't find 'em. So don't know if using another brand mixer is as low as the Yamaha.