generally, the red/white analog outputs of the CD player would go into a red/white analog input of your receiver which may be labelled CD, aux or some such.
The fixed will have the receiver have total control over it's output to your speakers If it's too loud, you can turn it down before it hits the receiver by using the variable output. Generally, the fixed output is fine.
Of course, having no idea exactly what receiver you're talking about may change things.