High Level Phono?
You are plugged into a Phono Input, but, just to be sure:
Some modern devices skipped a Phono PreAmp, and gave you an input labeled Phono, but, it is a High Level Input. It works with Turntables with built-in preamps, or, signals from a preamp already having boosted the signal up to 'line' level.
This type of high level phono input assumes you have already run the signal through a Phono 'RIAA' PreAmp which does two things: 1. boosts the signal up to line level about 1.0 volt, and very importantly, 2. re-adjusts the signal, because it was adjusted during recording.
If your setup skips the Phono RIAA PreAmp stage, the signal will definitely be too much highs and too little bass.
Your manual should give you the answer. Line Level is close to 1.0 volt.
Regards, Elliott