i have a DEH-2950MP Player in my car, as it has front AUX
i have tested that I copy a song from CD to USB
I have compare that song by playing it from CD
and then play it form USB --> AUX
I have found that THERE IS NO DIFFERENCE in the the sound quality
If you want to connect this player direct to Amplifier then result may differ,
To use this player I recommends to use Head unit's AUX option