Generally a CD player is supposed to be 2 volts (with an RCA interconnect) (With a XLR that would be 4 volts) . But many are more. The way to find the right setting is play music. adjust the volume to HALFWAY. if it is too loud at V 4 setting, move it to 8. If it is too soft at 4v try 2v
The 4v -8v is for XLR balanced. The 2V / 4v would be for the RCA, unbalanced)
also the Vitus may depend on it NOT OVERLOADING the input. If you set the ’v’ too low, (so the integrated makes it really loud at a low volume knob or control setting).. you may be overloading the input. (If that is hard to understand ,do not worry about it.) So you might want to start with the 4v setting and see where the volume control IS with the CD player input set to 4v.
Play Rock music, as it is mostly loud on the CD. Where Classical and Jazz may be only moderately loud. You want to listen to music which is using most of the CD loudness.
Most equipment is happy playing at 50% to 60%... Give you lots of room to play softer, or louder. I looked up a Vitus not knowing your model, and the Vitus I looked at has balanced inputs and RCA. So again I would start with the 4v and see how close the volume knob or control is to half way for pleasing level of playing Rock music with either RCA type or XLR type cables.