Only sure why to find out would be to measure the voltage around each Op Amp in the Phono Section to find out where the missing voltage is dropping. You'll need to take it a Tech, sounds like an easy repair.
When i had an original 2016 i had the same issues.
It turned out to be the switches on the rear panel (phono/line switches, Theres 4 of them)
Spray plenty of contact cleaner in and push them in and out several times.
just spray as much as you can into the switches and push them in and out several times.
A bit on the board wont hurt anything, just clean any excessive amount off with a cotton bud or something
Leave it a couple of minutes before powering back up.
There's not much else you can do unless you know how to measure voltage and interpret the results. Switches do have a mechanical life, so it may need replacing though you could bypass the switch to find out by using jumper cables, that's assuming the switch is at fault. Other than that, you'll need a tech and if i a recall correctly most components on there are surface mount which always makes repair a little more tricky.